[gtk/avoid-gl-texture-download] gsk: Avoid downloading GL textures when possible

[Matthias Clasen <matthiasc src gnome org>]

commit 9ca9f42452700b29ccbe40449127018d7077fc7c
Author: Matthias Clasen <mclasen redhat com>
Date:   Sat Sep 26 21:55:28 2020 -0400

    gsk: Avoid downloading GL textures when possible
    
    I found that the gears demo was spending 40% cpu
    downloading a GL texture every frame, only to
    upload it again to another context.
    
    While the GSK rendering and the GtkGLArea use different
    GL contexts, they are (usually) connected by sharing data
    with the same global context, so we can just use the
    texture without the download/upload dance. This brings
    gears down to < 10% cpu.

 gsk/gl/gskgldriver.c | 14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)
---
diff --git a/gsk/gl/gskgldriver.c b/gsk/gl/gskgldriver.c
index a383344eae..b1f8d6ea0e 100644
--- a/gsk/gl/gskgldriver.c
+++ b/gsk/gl/gskgldriver.c
@@ -521,8 +521,15 @@ gsk_gl_driver_get_texture_for_texture (GskGLDriver *self,
   if (GDK_IS_GL_TEXTURE (texture))
     {
       GdkGLContext *texture_context = gdk_gl_texture_get_context ((GdkGLTexture *)texture);
+      GdkGLContext *shared_context = gdk_gl_context_get_shared_context (self->gl_context);
 
-      if (texture_context != self->gl_context)
+      if (texture_context == self->gl_context ||
+          (gdk_gl_context_get_shared_context (texture_context) == shared_context && shared_context != NULL))
+        {
+          /* A GL texture from the same GL context is a simple task... */
+          return gdk_gl_texture_get_id ((GdkGLTexture *)texture);
+        }
+      else
         {
           cairo_surface_t *surface;
 
@@ -539,11 +546,6 @@ gsk_gl_driver_get_texture_for_texture (GskGLDriver *self,
 
           source_texture = downloaded_texture;
         }
-      else
-        {
-          /* A GL texture from the same GL context is a simple task... */
-          return gdk_gl_texture_get_id ((GdkGLTexture *)texture);
-        }
     }
   else
     {