gfx: Implement box-shadow per CSS-BACKGROUNDS.

components/gfx/paint_task.rs

@@ -509,6 +509,7 @@ impl WorkerThread {
                 font_ctx: &mut self.font_context,
                 page_rect: tile.page_rect,
                 screen_rect: tile.screen_rect,
+                clip_rect: None,
                 transient_clip_rect: None,
             };