support rendering of wider range of combinging characters by ftfont backend

* lisp/language/hebrew.el (hebrew-shape-gstring): If the font backend
supports rendering of combining characters, call
font-shape-gstring.

* src/font.c (Ffont_get): Handle `combining-capability' property.
(syms_of_font): New symbol ":combining-capability'.

* src/font.h (struct font_driver): New member combining_capability.

* src/ftfont.c: Include "category.h".
(ftfont_driver): Initialize combining_capability to
ftfont_combining_capability.
(ftfont_shape_by_flt): If OTF is null, try to find a suitable
FLT in advance.
(ftfont_combining_capability): New function.

(cherry picked from commit 536f48e9a2251b9e654ea974bd90ff2f40218753)

4 files changed, 72 insertions, 29 deletions

diff --git a/lisp/language/hebrew.el b/lisp/language/hebrew.el
index 498f01dd84e..ae8cc95c968 100644
--- a/lisp/language/hebrew.el
+++ b/lisp/language/hebrew.el
@@ -216,24 +216,26 @@ Bidirectional editing is supported.")))
 	  (setq idx 1 nglyphs nchars))
 	;; Now IDX is an index to the first non-precomposed glyph.
 	;; Adjust positions of the remaining glyphs artificially.
-	(setq base-width (lglyph-width (lgstring-glyph gstring 0)))
-	(while (< idx nglyphs)
-	  (setq glyph (lgstring-glyph gstring idx))
-	  (lglyph-set-from-to glyph 0 (1- nchars))
-	  (if (>= (lglyph-lbearing glyph) (lglyph-width glyph))
-	      ;; It seems that this glyph is designed to be rendered
-	      ;; before the base glyph.
-	      (lglyph-set-adjustment glyph (- base-width) 0 0)
-	    (if (>= (lglyph-lbearing glyph) 0)
-		;; Align the horizontal center of this glyph to the
-		;; horizontal center of the base glyph.
-		(let ((width (- (lglyph-rbearing glyph)
-				(lglyph-lbearing glyph))))
-		  (lglyph-set-adjustment glyph
-					 (- (/ (- base-width width) 2)
-					    (lglyph-lbearing glyph)
-					    base-width) 0 0))))
-	  (setq idx (1+ idx))))))
+        (if (font-get font :combining-capability)
+            (font-shape-gstring gstring)
+          (setq base-width (lglyph-width (lgstring-glyph gstring 0)))
+          (while (< idx nglyphs)
+            (setq glyph (lgstring-glyph gstring idx))
+            (lglyph-set-from-to glyph 0 (1- nchars))
+            (if (>= (lglyph-lbearing glyph) (lglyph-width glyph))
+                ;; It seems that this glyph is designed to be rendered
+                ;; before the base glyph.
+                (lglyph-set-adjustment glyph (- base-width) 0 0)
+              (if (>= (lglyph-lbearing glyph) 0)
+                  ;; Align the horizontal center of this glyph to the
+                  ;; horizontal center of the base glyph.
+                  (let ((width (- (lglyph-rbearing glyph)
+                                  (lglyph-lbearing glyph))))
+                    (lglyph-set-adjustment glyph
+                                           (- (/ (- base-width width) 2)
+                                              (lglyph-lbearing glyph)
+                                              base-width) 0 0))))
+            (setq idx (1+ idx)))))))
     gstring))
 
 (let* ((base "[\u05D0-\u05F2]")
diff --git a/src/font.c b/src/font.c
index 6af9e7cde1f..039493bcbea 100644
--- a/src/font.c
+++ b/src/font.c
@@ -4036,7 +4036,13 @@ The value of :otf is a cons (GSUB . GPOS) where GSUB and GPOS are lists
 representing the OpenType features supported by the font by this form:
   ((SCRIPT (LANGSYS FEATURE ...) ...) ...)
 SCRIPT, LANGSYS, and FEATURE are all symbols representing OpenType
-Layout tags.  */)
+Layout tags.
+
+In addition to the keys listed abobe, the following keys are reserved
+for the specific meanings as below:
+
+The value of :combining-capability is non-nil if the font-backend of
+FONT supports rendering of combining characters for non-OTF fonts.  */)
   (Lisp_Object font, Lisp_Object key)
 {
   int idx;
@@ -4051,14 +4057,22 @@ Layout tags.  */)
   if (idx >= 0 && idx < FONT_EXTRA_INDEX)
     return AREF (font, idx);
   val = Fassq (key, AREF (font, FONT_EXTRA_INDEX));
-  if (NILP (val) && EQ (key, QCotf) && FONT_OBJECT_P (font))
+  if (NILP (val) && FONT_OBJECT_P (font))
     {
       struct font *fontp = XFONT_OBJECT (font);
 
-      if (fontp->driver->otf_capability)
-	val = fontp->driver->otf_capability (fontp);
-      else
-	val = Fcons (Qnil, Qnil);
+      if (EQ (key, QCotf))
+	{
+	  if (fontp->driver->otf_capability)
+	    val = fontp->driver->otf_capability (fontp);
+	  else
+	    val = Fcons (Qnil, Qnil);
+	}
+      else if (EQ (key, QCcombining_capability))
+	{
+	  if (fontp->driver->combining_capability)
+	    val = fontp->driver->combining_capability (fontp);
+	}
     }
   else
     val = Fcdr (val);
@@ -5290,6 +5304,7 @@ syms_of_font (void)
   DEFSYM (QCscalable, ":scalable");
   DEFSYM (QCavgwidth, ":avgwidth");
   DEFSYM (QCfont_entity, ":font-entity");
+  DEFSYM (QCcombining_capability, ":combining-capability");
 
   /* Symbols representing values of font spacing property.  */
   DEFSYM (Qc, "c");
diff --git a/src/font.h b/src/font.h
index ba208e3c27d..36fe51ad319 100644
--- a/src/font.h
+++ b/src/font.h
@@ -757,6 +757,12 @@ struct font_driver
   bool (*cached_font_ok) (struct frame *f,
                           Lisp_Object font_object,
                           Lisp_Object entity);
+
+  /* Optional
+
+     Return non-nil if the driver support rendering of combining
+     characters for FONT according to Unicode combining class.  */
+  Lisp_Object (*combining_capability) (struct font *font);
 };
 
 
diff --git a/src/ftfont.c b/src/ftfont.c
index 8412dd0e286..bb8af96d7b1 100644
--- a/src/ftfont.c
+++ b/src/ftfont.c
@@ -30,6 +30,7 @@ along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.  */
 #include "dispextern.h"
 #include "character.h"
 #include "charset.h"
+#include "category.h"
 #include "composite.h"
 #include "font.h"
 #include "ftfont.h"
@@ -81,6 +82,8 @@ static Lisp_Object ftfont_lookup_cache (Lisp_Object,
 
 static void ftfont_filter_properties (Lisp_Object font, Lisp_Object alist);
 
+static Lisp_Object ftfont_combining_capability (struct font *);
+
 #define SYMBOL_FcChar8(SYM) (FcChar8 *) SDATA (SYMBOL_NAME (SYM))
 
 static struct
@@ -547,6 +550,10 @@ struct font_driver ftfont_driver =
 #endif
 
     ftfont_filter_properties, /* filter_properties */
+
+    NULL,			/* cached_font_ok */
+
+    ftfont_combining_capability,
   };
 
 static Lisp_Object
@@ -2533,7 +2540,7 @@ ftfont_shape_by_flt (Lisp_Object lgstring, struct font *font,
 
   len = i;
 
-  if (with_variation_selector)
+  if (otf && with_variation_selector)
     {
       setup_otf_gstring (len);
       for (i = 0; i < len; i++)
@@ -2583,14 +2590,19 @@ ftfont_shape_by_flt (Lisp_Object lgstring, struct font *font,
   flt_font_ft.otf = otf;
   flt_font_ft.matrix = matrix->xx != 0 ? matrix : 0;
 
-  if (1 < len)
+  if (1 < len || ! otf)
     {
       /* A little bit ad hoc.  Perhaps, shaper must get script and
 	 language information, and select a proper flt for them
 	 here.  */
       int c1 = LGLYPH_CHAR (LGSTRING_GLYPH (lgstring, 1));
-      if (0x300 <= c1 && c1 <= 0x36F)
+      if (CHAR_HAS_CATEGORY (c1, '^'))
 	flt = mflt_get (msymbol ("combining"));
+      else if (! otf)
+	flt = mflt_find (LGLYPH_CHAR (LGSTRING_GLYPH (lgstring, 0)),
+			 &flt_font_ft.flt_font);
+      if (! flt)
+	return make_number (0);
     }
 
   MFLTGlyphFT *glyphs = (MFLTGlyphFT *) gstring.glyphs;
@@ -2675,8 +2687,6 @@ ftfont_shape (Lisp_Object lgstring)
   struct ftfont_info *ftfont_info = (struct ftfont_info *) font;
   OTF *otf = ftfont_get_otf (ftfont_info);
 
-  if (! otf)
-    return make_number (0);
   return ftfont_shape_by_flt (lgstring, font, ftfont_info->ft_size->face, otf,
 			      &ftfont_info->matrix);
 }
@@ -2750,6 +2760,16 @@ ftfont_filter_properties (Lisp_Object font, Lisp_Object alist)
 }
 
 
+static Lisp_Object
+ftfont_combining_capability (struct font *font)
+{
+#ifdef HAVE_M17N_FLT
+  return Qt;
+#else
+  return Qnil;
+#endif
+}
+
 void
 syms_of_ftfont (void)
 {