st-theme-node: Add method to invalidate drawing state

StThemeNode caches its resources aggressively to keep the required
work on paint to a minimum - right now, resources are only recreated
on allocation changes.
In order to update the background-image property correctly when the
underlying file changes, resources need to be recreated without a
size change, so add an explicit method for that.

https://bugzilla.gnome.org/show_bug.cgi?id=679268

src/st/st-theme-node-drawing.c

@@ -2088,3 +2088,10 @@ st_theme_node_copy_cached_paint_state (StThemeNode *node,
     if (other->corner_material[corner_id])
       node->corner_material[corner_id] = cogl_handle_ref (other->corner_material[corner_id]);
 }
+
+void
+st_theme_node_invalidate_paint_state (StThemeNode *node)
+{
+  node->alloc_width = 0;
+  node->alloc_height = 0;
+}

src/st/st-theme-node.h

@@ -244,6 +244,7 @@ void st_theme_node_paint (StThemeNode            *node,
 
 void st_theme_node_copy_cached_paint_state (StThemeNode *node,
                                             StThemeNode *other);
+void st_theme_node_invalidate_paint_state  (StThemeNode *node);
 
 G_END_DECLS