cursor-renderer/native: Let the backend realize cursor sprite itself

It knows better when it's needed. For now, just do it just as before,
before drawing. Eventually, we can conditionalize where to realize
depending on the cursor sprite position.

https://gitlab.gnome.org/GNOME/mutter/issues/77

src/backends/meta-cursor-renderer.c

@@ -283,13 +283,3 @@ meta_cursor_renderer_get_cursor (MetaCursorRenderer *renderer)
 
   return priv->displayed_cursor;
 }
-
-void
-meta_cursor_renderer_realize_cursor_sprite (MetaCursorRenderer *renderer,
-                                            MetaCursorSprite   *cursor_sprite)
-{
-  MetaCursorRendererClass *renderer_class = META_CURSOR_RENDERER_GET_CLASS (renderer);
-
-  if (renderer_class->realize_cursor_sprite)
-    renderer_class->realize_cursor_sprite (renderer, cursor_sprite);
-}