Move LayerBuffer cache to the compositor

Now that NativeDisplay can be shared between the compositor and the
paint task, we can move the LayerBuffer cache to the compositor. This
allows surfaces to be potentially reused between different paint tasks
and will eventually allow OpenGL contexts to be preserved between
instances of GL rasterization.

components/compositing/lib.rs

@@ -3,6 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #![feature(box_syntax)]
+#![feature(iter_cmp)]
 #![feature(slice_bytes)]
 #![feature(vec_push_all)]
 
@@ -43,6 +44,7 @@ pub use constellation::Constellation;
 
 pub mod compositor_task;
 
+mod buffer_map;
 mod compositor_layer;
 mod compositor;
 mod headless;