wayland: Don't allocate a full size SHM buffer when drawing using OpenGL

Before this patch, we'd always allocate a full size SHM buffer via
the wl_shm_pool, even though it would never be used. Instead allocate a
logical 1x1 cairo image surface.

https://bugzilla.gnome.org/show_bug.cgi?id=745076

gdk/wayland/gdkdisplay-wayland.c

@@ -834,6 +834,12 @@ _gdk_wayland_shm_surface_get_busy (cairo_surface_t *surface)
   return data->busy;
 }
 
+gboolean
+_gdk_wayland_is_shm_surface (cairo_surface_t *surface)
+{
+  return cairo_surface_get_user_data (surface, &gdk_wayland_cairo_key) != NULL;
+}
+
 GdkWaylandSelection *
 gdk_wayland_display_get_selection (GdkDisplay *display)
 {