[gtk+/gtk-3-20] gdk: Provide safe fallback for creating GL contexts

[Matthias Clasen <matthiasc src gnome org>]

commit 6b2a98489284cdd4c10cc9ab550be5e165694066
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Mon May 23 08:34:39 2016 +0100

    gdk: Provide safe fallback for creating GL contexts
    
    If the backend does not override GdkWindowClass.create_gl_context()
    then we should provide a fallback that gives an error, instead of
    crashing.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=766782

 gdk/gdkwindow.c |   17 +++++++++++++----
 1 files changed, 13 insertions(+), 4 deletions(-)
---
diff --git a/gdk/gdkwindow.c b/gdk/gdkwindow.c
index 66ebb9d..b7f8a96 100644
--- a/gdk/gdkwindow.c
+++ b/gdk/gdkwindow.c
@@ -2769,11 +2769,20 @@ gdk_window_get_paint_gl_context (GdkWindow  *window,
 
   if (window->impl_window->gl_paint_context == NULL)
     {
+      GdkWindowImplClass *impl_class = GDK_WINDOW_IMPL_GET_CLASS (window->impl);
+
+      if (impl_class->create_gl_context == NULL)
+        {
+          g_set_error_literal (error, GDK_GL_ERROR, GDK_GL_ERROR_NOT_AVAILABLE,
+                               _("The current backend does not support OpenGL"));
+          return NULL;
+        }
+
       window->impl_window->gl_paint_context =
-        GDK_WINDOW_IMPL_GET_CLASS (window->impl)->create_gl_context (window->impl_window,
-                                                                     TRUE,
-                                                                     NULL,
-                                                                     &internal_error);
+        impl_class->create_gl_context (window->impl_window,
+                                       TRUE,
+                                       NULL,
+                                       &internal_error);
     }
 
   if (internal_error != NULL)