Issue #2073 - m-c 1343341: Infrastructure necessary to allow discarding of animated images (squashed)

Includes squashed changes of:
 - m-c 1317907: Refactor FrameAnimator::GetCompositedFrame to be a bit simpler
 - m-c 1351434: bugfix
 - m-c 686905: Enable the pref image.mem.animated.discardable to allow discarding of animated images

image/VectorImage.cpp

@@ -1113,7 +1113,7 @@ VectorImage::RequestDiscard()
 }
 
 void
-VectorImage::OnSurfaceDiscarded()
+VectorImage::OnSurfaceDiscarded(const SurfaceKey& aSurfaceKey)
 {
   MOZ_ASSERT(mProgressTracker);