Bug 1496413 - Pass texture clients to paint thread when we discard them. r=nical

--HG--
extra : rebase_source : e10d6a0881618742b1fd767e19d128fd38ff4312
extra : histedit_source : 4da39991453357d7d06f41f8f3f6461f3838faa8

gfx/layers/client/SingleTiledContentClient.cpp

@@ -227,6 +227,12 @@ void ClientSingleTiledLayerBuffer::PaintThebes(
       task->mCapture = backBuffer->mCapture;
       task->mTarget = backBuffer->mBackBuffer;
       task->mClients = std::move(backBuffer->mTextureClients);
+      if (discardedFrontBuffer) {
+        task->mClients.AppendElement(discardedFrontBuffer);
+      }
+      if (discardedFrontBufferOnWhite) {
+        task->mClients.AppendElement(discardedFrontBufferOnWhite);
+      }
 
       // The target is an alias for the capture, and the paint thread expects
       // to be the only one with a reference to the capture