Bug 1391793 - Save the given ImageContainer in WebRenderImageData so that it is not recreated. r=jrmuizel

A RasterImage's ImageContainer is only retained as a weak pointer in the
RasterImage itself. With WebRender and display items, we no longer kept
a strong reference to said image container, and as such it got recreated
every time it had to repaint (e.g. after a scroll). This would
unnecessarily reset/update the generation counter such that the
ImageClient would decide it needs to retransmit the image data to the
GPU process. By keeping a strong reference to the container that we
don't otherwise use, we avoid this situation.

gfx/layers/wr/WebRenderUserData.cpp

@@ -62,6 +62,10 @@ WebRenderImageData::UpdateImageKey(ImageContainer* aContainer, bool aForceUpdate
   CreateImageClientIfNeeded();
   CreateExternalImageIfNeeded();
 
+  if (mContainer != aContainer) {
+    mContainer = aContainer;
+  }
+
   if (!mImageClient || !mExternalImageId) {
     return Nothing();
   }