layout: Paint stacking contexts' overflow areas properly.

This was making `box-shadow` not show up in many cases, in particular,
but the effects were not limited to that.

components/gfx/display_list/optimizer.rs

@@ -59,7 +59,8 @@ impl DisplayListOptimizer {
                                              mut stacking_contexts: I)
                                              where I: Iterator<&'a Arc<StackingContext>> {
         for stacking_context in stacking_contexts {
-            if self.visible_rect.intersects(&stacking_context.bounds) {
+            let overflow = stacking_context.overflow.translate(&stacking_context.bounds.origin);
+            if self.visible_rect.intersects(&overflow) {
                 result_list.push_back((*stacking_context).clone())
             }
         }