Fix display when a font claims large values of ascent and descent

This fixes bug#20628.

* src/xdisp.c (get_phys_cursor_geometry): Correct the Y
coordinate of a hollow cursor glyph when the original glyph's
ascent is too small.
(get_font_ascent_descent, normal_char_ascent_descent)
(normal_char_height): New functions.
(handle_single_display_spec, append_space_for_newline)
(calc_pixel_width_or_height, produce_stretch_glyph)
(calc_line_height_property): Use normal_char_ascent_descent and
normal_char_height.
(x_produce_glyphs): When font-global values of ascent and descent
are too large, use per-character glyph metrics instead, if
possible.  But don't allow the glyph row's ascent and descent
values become smaller than the values from the metrics of the
font's "normal" character.

* src/xftfont.c (xftfont_draw):
* src/w32font.c (w32font_draw): Correct the values of ascent and
descent used to draw glyphless characters' hex code in a box.

* src/xterm.c (x_draw_glyph_string_background):
* src/xdisp.c (x_produce_glyphs):
* src/w32term.c (x_draw_glyph_string_background):
* src/nsterm.m (ns_maybe_dumpglyphs_background): Use FONT_TOO_HIGH
to detect fonts whose global ascent and descent values are too
large to be used in layout decision, and redraw the background
when that happens.

* src/dispextern.h (FONT_TOO_HIGH): New macro.
(get_font_ascent_descent): Add prototype.

* src/xterm.c (x_new_font):
* src/w32term.c (x_new_font):
* src/nsterm.m (x_new_font):
* src/font.c (font_open_entity):
* src/composite.c (composition_gstring_width): Use
get_font_ascent_descent to obtain reasonable values for ascent and
descent of a font.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/composite.c b/src/composite.c
index 1c9c62bf910..1c18165599d 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -732,9 +732,11 @@ composition_gstring_width (Lisp_Object gstring, ptrdiff_t from, ptrdiff_t to,
       if (FONT_OBJECT_P (font_object))
 	{
 	  struct font *font = XFONT_OBJECT (font_object);
+	  int font_ascent, font_descent;
 
-	  metrics->ascent = font->ascent;
-	  metrics->descent = font->descent;
+	  get_font_ascent_descent (font, &font_ascent, &font_descent);
+	  metrics->ascent = font_ascent;
+	  metrics->descent = font_descent;
 	}
       else
 	{