Use real clips when generating scroll roots

This is the first step toward removing inherited clips in favor of
scroll roots for handling overflow and CSS clipping. This will allow us
to more easily handle elements that should not be clipped. While we are
still using inherited clips here, we now properly clip some types of
content that wasn't clipped before.

components/gfx/display_list/mod.rs

@@ -508,7 +508,7 @@ pub struct ScrollRoot {
     pub parent_id: ScrollRootId,
 
     /// The position of this scroll root's frame in the parent stacking context.
-    pub clip: Rect<Au>,
+    pub clip: ClippingRegion,
 
     /// The rect of the contents that can be scrolled inside of the scroll root.
     pub content_rect: Rect<Au>,