renderer/x11: Enable GPU memory purge error extension if available

This was done by the clutter X11 backend before prior to introducing
MetaRenderer, but during that work, enabling of said extension was lost.
Let's turn it on again.

https://bugzilla.gnome.org/show_bug.cgi?id=739178

src/backends/x11/meta-backend-x11.c

@@ -705,8 +705,6 @@ meta_backend_x11_init (MetaBackendX11 *x11)
    */
   XInitThreads();
 
-  clutter_x11_request_reset_on_video_memory_purge ();
-
   /* We do X11 event retrieval ourselves */
   clutter_x11_disable_event_retrieval ();
 }

src/backends/x11/meta-renderer-x11.c

@@ -74,6 +74,7 @@ meta_renderer_x11_create_cogl_renderer (MetaRenderer *renderer)
   cogl_renderer_set_custom_winsys (cogl_renderer, get_x11_cogl_winsys_vtable,
                                    NULL);
   cogl_xlib_renderer_set_foreign_display (cogl_renderer, xdisplay);
+  cogl_xlib_renderer_request_reset_on_video_memory_purge (cogl_renderer, TRUE);
 
   /* Set up things so that if the INTEL_swap_event extension is not present,
    * but the driver is known to have good thread support, we use an extra