cogl/gl-framebuffer: Move stereo mode state flushing to backend

It's currently only handled by a surface backend framebuffer (assuming
the right GLX extensions are available). While it's theoretically
possible to do the same with the offcreen by having multiple textures,
it's not supported, so leave the FBO variant with a single warning if we
end up there.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1514>

4 files changed, 63 insertions, 33 deletions

diff --git a/cogl/cogl/driver/gl/cogl-framebuffer-gl-private.h b/cogl/cogl/driver/gl/cogl-framebuffer-gl-private.h
index 4bdd076a6..3f3f6b7c8 100644
--- a/cogl/cogl/driver/gl/cogl-framebuffer-gl-private.h
+++ b/cogl/cogl/driver/gl/cogl-framebuffer-gl-private.h
@@ -49,6 +49,8 @@ struct _CoglGlFramebufferClass
 
   void (* bind) (CoglGlFramebuffer *gl_framebuffer,
                  GLenum             target);
+
+  void (* flush_stereo_mode_state) (CoglGlFramebuffer *gl_framebuffer);
 };
 
 void
diff --git a/cogl/cogl/driver/gl/cogl-framebuffer-gl.c b/cogl/cogl/driver/gl/cogl-framebuffer-gl.c
index d7407bee5..6e807fa71 100644
--- a/cogl/cogl/driver/gl/cogl-framebuffer-gl.c
+++ b/cogl/cogl/driver/gl/cogl-framebuffer-gl.c
@@ -194,40 +194,10 @@ cogl_gl_framebuffer_flush_front_face_winding_state (CoglGlFramebuffer *gl_frameb
 static void
 cogl_gl_framebuffer_flush_stereo_mode_state (CoglGlFramebuffer *gl_framebuffer)
 {
-  CoglFramebufferDriver *driver = COGL_FRAMEBUFFER_DRIVER (gl_framebuffer);
-  CoglFramebuffer *framebuffer =
-    cogl_framebuffer_driver_get_framebuffer (driver);
-  CoglContext *ctx = cogl_framebuffer_get_context (framebuffer);
-  GLenum draw_buffer = GL_BACK;
-
-  if (COGL_IS_OFFSCREEN (framebuffer))
-    return;
-
-  if (!ctx->glDrawBuffer)
-    return;
+  CoglGlFramebufferClass *klass =
+    COGL_GL_FRAMEBUFFER_GET_CLASS (gl_framebuffer);
 
-  /* The one-shot default draw buffer setting in _cogl_framebuffer_gl_bind
-   * must have already happened. If not it would override what we set here. */
-  g_assert (ctx->was_bound_to_onscreen);
-
-  switch (cogl_framebuffer_get_stereo_mode (framebuffer))
-    {
-    case COGL_STEREO_BOTH:
-      draw_buffer = GL_BACK;
-      break;
-    case COGL_STEREO_LEFT:
-      draw_buffer = GL_BACK_LEFT;
-      break;
-    case COGL_STEREO_RIGHT:
-      draw_buffer = GL_BACK_RIGHT;
-      break;
-    }
-
-  if (ctx->current_gl_draw_buffer != draw_buffer)
-    {
-      GE (ctx, glDrawBuffer (draw_buffer));
-      ctx->current_gl_draw_buffer = draw_buffer;
-    }
+  klass->flush_stereo_mode_state (gl_framebuffer);
 }
 
 void
diff --git a/cogl/cogl/driver/gl/cogl-gl-framebuffer-back.c b/cogl/cogl/driver/gl/cogl-gl-framebuffer-back.c
index 4788bb9be..cc2510fe5 100644
--- a/cogl/cogl/driver/gl/cogl-gl-framebuffer-back.c
+++ b/cogl/cogl/driver/gl/cogl-gl-framebuffer-back.c
@@ -227,6 +227,42 @@ cogl_gl_framebuffer_back_bind (CoglGlFramebuffer *gl_framebuffer,
     }
 }
 
+static void
+cogl_gl_framebuffer_back_flush_stereo_mode_state (CoglGlFramebuffer *gl_framebuffer)
+{
+  CoglFramebufferDriver *driver = COGL_FRAMEBUFFER_DRIVER (gl_framebuffer);
+  CoglFramebuffer *framebuffer =
+    cogl_framebuffer_driver_get_framebuffer (driver);
+  CoglContext *ctx = cogl_framebuffer_get_context (framebuffer);
+  GLenum draw_buffer = GL_BACK;
+
+  if (!ctx->glDrawBuffer)
+    return;
+
+  /* The one-shot default draw buffer setting in _cogl_framebuffer_gl_bind
+   * must have already happened. If not it would override what we set here. */
+  g_assert (ctx->was_bound_to_onscreen);
+
+  switch (cogl_framebuffer_get_stereo_mode (framebuffer))
+    {
+    case COGL_STEREO_BOTH:
+      draw_buffer = GL_BACK;
+      break;
+    case COGL_STEREO_LEFT:
+      draw_buffer = GL_BACK_LEFT;
+      break;
+    case COGL_STEREO_RIGHT:
+      draw_buffer = GL_BACK_RIGHT;
+      break;
+    }
+
+  if (ctx->current_gl_draw_buffer != draw_buffer)
+    {
+      GE (ctx, glDrawBuffer (draw_buffer));
+      ctx->current_gl_draw_buffer = draw_buffer;
+    }
+}
+
 CoglGlFramebufferBack *
 cogl_gl_framebuffer_back_new (CoglFramebuffer                    *framebuffer,
                               const CoglFramebufferDriverConfig  *driver_config,
@@ -262,4 +298,6 @@ cogl_gl_framebuffer_back_class_init (CoglGlFramebufferBackClass *klass)
   driver_class->discard_buffers = cogl_gl_framebuffer_back_discard_buffers;
 
   gl_framebuffer_class->bind = cogl_gl_framebuffer_back_bind;
+  gl_framebuffer_class->flush_stereo_mode_state =
+    cogl_gl_framebuffer_back_flush_stereo_mode_state;
 }
diff --git a/cogl/cogl/driver/gl/cogl-gl-framebuffer-fbo.c b/cogl/cogl/driver/gl/cogl-gl-framebuffer-fbo.c
index 0a9bbd680..e1cd38792 100644
--- a/cogl/cogl/driver/gl/cogl-gl-framebuffer-fbo.c
+++ b/cogl/cogl/driver/gl/cogl-gl-framebuffer-fbo.c
@@ -214,6 +214,24 @@ cogl_gl_framebuffer_fbo_bind (CoglGlFramebuffer *gl_framebuffer,
   GE (ctx, glBindFramebuffer (target, gl_framebuffer_fbo->gl_fbo.fbo_handle));
 }
 
+static void
+cogl_gl_framebuffer_fbo_flush_stereo_mode_state (CoglGlFramebuffer *gl_framebuffer)
+{
+  CoglFramebufferDriver *driver = COGL_FRAMEBUFFER_DRIVER (gl_framebuffer);
+  CoglFramebuffer *framebuffer =
+    cogl_framebuffer_driver_get_framebuffer (driver);
+
+  switch (cogl_framebuffer_get_stereo_mode (framebuffer))
+    {
+    case COGL_STEREO_BOTH:
+      break;
+    case COGL_STEREO_LEFT:
+    case COGL_STEREO_RIGHT:
+      g_warn_if_reached ();
+      break;
+    }
+}
+
 static GList *
 try_creating_renderbuffers (CoglContext                *ctx,
                             int                         width,
@@ -636,4 +654,6 @@ cogl_gl_framebuffer_fbo_class_init (CoglGlFramebufferFboClass *klass)
   driver_class->discard_buffers = cogl_gl_framebuffer_fbo_discard_buffers;
 
   gl_framebuffer_class->bind = cogl_gl_framebuffer_fbo_bind;
+  gl_framebuffer_class->flush_stereo_mode_state =
+    cogl_gl_framebuffer_fbo_flush_stereo_mode_state;
 }