Delegated renderer: Put textures on those quads.

A few changes are necessary to allow fetching textures provided by
the render processes through IPC and bound to their respective GL
context in the GPU process and use them in the QtQuick scene graph.

- Remove the plain color test textures.
- Allow setting the QtQuick QOpenGLContext's handle as the share
  context for all context set as shared in the Chromium GPU process.
  We do this by letting the GpuChannelManager ask us for a
  ShareGroup instance responsible for returning a sharing GL context
  handle.
- Fetch texture IDs from the MailboxManager used by the GPU process
  using the Mailbox given to us in the DelegatedFrameData.
  This is the same mechanism used by Chromium to share textures
  between "client" GL contexts.
- Keep the QtQuick scene graph threads and Chromium in-process GPU
  thread separate. The complicated part of merging those two
  rendering pipelines on the same thread is that it would force Qt
  to also use only one thread for rende...