Have BufferMap store NativeSurfaces and rename to SurfaceMap

We currently store LayerBuffers, because previously NativeSurfaces did
not record their own size. Now we can store NativeSurfaces directly,
which saves a bit of space in the surface cache and allows us to create
LayerBuffers only in the PaintTask.

This also means that instead of sending cached LayerBuffers, the
compositor can just send cached NativeSurfaces to the PaintTask.

components/compositing/headless.rs

@@ -111,7 +111,7 @@ impl CompositorEventListener for NullCompositor {
             Msg::IsReadyToSaveImageReply(..) => {}
             Msg::NewFavicon(..) => {}
             Msg::HeadParsed => {}
-            Msg::ReturnUnusedLayerBuffers(..) => {}
+            Msg::ReturnUnusedNativeSurfaces(..) => {}
             Msg::CollectMemoryReports(..) => {}
         }
         true