compositor: Handle GL video memory purged errors

Emit a signal so that interested parties can recreate their FBOs and
queue a full scene graph redraw to ensure we don't end up showing
graphical artifacts.

This relies on the GL driver supporting the
NV_robustness_video_memory_purge extension and cogl creating a
suitable GL context. For now we only make use of it with the X backend
since the only driver with which this is useful is NVIDIA.

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

src/core/display.c

@@ -125,6 +125,7 @@ enum
   SHOW_RESTART_MESSAGE,
   RESTART,
   SHOW_RESIZE_POPUP,
+  GL_VIDEO_MEMORY_PURGED,
   LAST_SIGNAL
 };
 
@@ -342,6 +343,14 @@ meta_display_class_init (MetaDisplayClass *klass)
                   G_TYPE_BOOLEAN, 4,
                   G_TYPE_BOOLEAN, META_TYPE_RECTANGLE, G_TYPE_INT, G_TYPE_INT);
 
+  display_signals[GL_VIDEO_MEMORY_PURGED] =
+    g_signal_new ("gl-video-memory-purged",
+                  G_TYPE_FROM_CLASS (klass),
+                  G_SIGNAL_RUN_LAST,
+                  0,
+                  NULL, NULL, NULL,
+                  G_TYPE_NONE, 0);
+
   g_object_class_install_property (object_class,
                                    PROP_FOCUS_WINDOW,
                                    g_param_spec_object ("focus-window",