[gtk/readonly-all-the-things] gl: Add some comments

[Matthias Clasen <matthiasc src gnome org>]

commit b90801ec2b753bd6f4fbcb84e3686374973b3590
Author: Matthias Clasen <mclasen redhat com>
Date:   Tue Oct 22 02:27:28 2019 -0400

    gl: Add some comments

 gsk/gl/gskgldriver.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/gsk/gl/gskgldriver.c b/gsk/gl/gskgldriver.c
index 321681d507..02132fc463 100644
--- a/gsk/gl/gskgldriver.c
+++ b/gsk/gl/gskgldriver.c
@@ -46,8 +46,8 @@ struct _GskGLDriver
 
   Fbo default_fbo;
 
-  GHashTable *textures;
-  GHashTable *pointer_textures;
+  GHashTable *textures;         /* texture_id -> Texture */
+  GHashTable *pointer_textures; /* pointer -> texture_id */
 
   const Texture *bound_source_texture;