Fixes a number of race conditions and reliability issues with reftests and compositor.

The basic idea is it's safe to output an image for reftest by testing:
 - That the compositor doesn't have any animations active.
 - That the compositor is not waiting on any outstanding paint messages to arrive.
 - That the script tasks are "idle" and therefore won't cause reflow.
    - This currently means page loaded, onload fired, reftest-wait not active, first reflow triggered.
    - It could easily be expanded to handle pending timers etc.
 - That the "epoch" that the layout tasks have last laid out after script went idle, is reflected by the compositor in all visible layers for that pipeline.

components/compositing/headless.rs

@@ -92,16 +92,12 @@ impl CompositorEventListener for NullCompositor {
             // we'll notice and think about whether it needs a response, like
             // SetFrameTree.
 
-            Msg::CreateOrUpdateBaseLayer(..) |
-            Msg::CreateOrUpdateDescendantLayer(..) |
+            Msg::InitializeLayersForPipeline(..) |
             Msg::SetLayerRect(..) |
             Msg::AssignPaintedBuffers(..) |
-            Msg::ChangeReadyState(..) |
-            Msg::ChangePaintState(..) |
             Msg::ChangeRunningAnimationsState(..) |
             Msg::ScrollFragmentPoint(..) |
             Msg::LoadComplete |
-            Msg::PaintMsgDiscarded(..) |
             Msg::ScrollTimeout(..) |
             Msg::RecompositeAfterScroll |
             Msg::ChangePageTitle(..) |
@@ -110,7 +106,8 @@ impl CompositorEventListener for NullCompositor {
             Msg::SetCursor(..) |
             Msg::ViewportConstrained(..) => {}
             Msg::CreatePng(..) |
-            Msg::PaintTaskExited(..) => {}
+            Msg::PaintTaskExited(..) |
+            Msg::IsReadyToSaveImageReply(..) => {}
         }
         true
     }