layout: Implement basic overflow: scroll functionality.

Known issues:

* Display list optimization can sometimes optimize out elements that
  should be shown. This affects the Enyo demo.

* The `overflow: scroll` container doesn't clip the inner layer properly
  when borders, border radius, etc. are present.

* `overflow-x: scroll` and `overflow-y: scroll` don't work individually;
  elements are scrolled all at once.

* Scrolling only works on absolutely-positioned elements.

components/msg/compositor_msg.rs

@@ -88,6 +88,8 @@ pub struct LayerProperties {
     pub perspective: Matrix4,
     /// Whether this layer establishes a new 3d rendering context.
     pub establishes_3d_context: bool,
+    /// Whether this layer scrolls its overflow area.
+    pub scrolls_overflow_area: bool,
 }
 
 /// The interface used by the painter to acquire draw targets for each paint frame and