st-theme-node: Don't update resources on each paint

Up to commit 318283fc70, resources were only updated when
the allocated size had changed. There is no good reason to change
this for theme nodes without box shadows.

https://bugzilla.gnome.org/show_bug.cgi?id=703909

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

@@ -2397,7 +2397,7 @@ st_theme_node_paint (StThemeNode           *node,
 
       node->rendered_once = TRUE;
     }
-  else
+  else if (state->alloc_width != width || state->alloc_height != height)
     st_theme_node_update_resources (state, node, width, height);
 
   /* Rough notes about the relationship of borders and backgrounds in CSS3;