Don't copy over regions that will be painted in this frame (bug 1399692 part 8, r=bas)

We do the same in FinalizeFrame, so we should do it here.

MozReview-Commit-ID: JTKDj8yrtI2

gfx/layers/client/ContentClient.cpp

@@ -253,12 +253,16 @@ ContentClient::BeginPaint(PaintedLayer* aLayer,
 
     // If we have an existing front buffer, copy it into the new back buffer
     if (RefPtr<RotatedBuffer> frontBuffer = GetFrontBuffer()) {
+      nsIntRegion updateRegion = newBuffer->BufferRect();
+      updateRegion.Sub(updateRegion, result.mRegionToDraw);
+
+      if (!updateRegion.IsEmpty()) {
         RefPtr<CapturedBufferState> bufferState = new CapturedBufferState();
 
         bufferState->mBufferCopy = Some(CapturedBufferState::Copy {
           frontBuffer->ShallowCopy(),
           newBuffer->ShallowCopy(),
-        newBuffer->BufferRect(),
+          updateRegion.GetBounds(),
         });
 
         // If we're async painting then return the buffer state to
@@ -272,6 +276,7 @@ ContentClient::BeginPaint(PaintedLayer* aLayer,
             return result;
           }
         }
+      }
 
       if (dest.mMustRemoveFrontBuffer) {
         Clear();