Bug 1143575. Exit composition early if nothing is invalid. r=mattwoodrow

We need this change so that when ImageHost has a next image to display
more than one composition-interval in the future, we skip the actual
compositing work in those intermediate composition(s) if nothing else
has changed.

This change is a little bit scary since it breaks any code that was
previously assuming ScheduleComposition would actually update the screen.
However, that code was already broken for BasicCompositor.

--HG--
extra : commitid : FUFFaJDSwfu
extra : rebase_source : eb2d5fadd5dbd417e7a25c05b148bd8964e8c2a3

gfx/layers/composite/LayerManagerComposite.cpp

@@ -283,7 +283,12 @@ LayerManagerComposite::EndTransaction(const TimeStamp& aTimeStamp,
     mInvalidRegion.Or(mInvalidRegion, mRenderBounds);
   }
 
-  if (mRoot && !(aFlags & END_NO_IMMEDIATE_REDRAW)) {
+  if (mInvalidRegion.IsEmpty() && !mTarget) {
+    // Composition requested, but nothing has changed. Don't do any work.
+    return;
+  }
+
+ if (mRoot && !(aFlags & END_NO_IMMEDIATE_REDRAW)) {
     MOZ_ASSERT(!aTimeStamp.IsNull());
     // Set composition timestamp here because we need it in
     // ComputeEffectiveTransforms (so the correct video frame size is picked)