We would like to stop exposing rx::RenderTarget to any GL-layer
code, like the Context, or Framebuffer. Then we can pull any
necessary information inside the Renderer back-ends.

BUG=angle:732

Change-Id: Ifa35218f097c528e616d732bda4bcd8e5f1c3843
Reviewed-on: https://chromium-review.googlesource.com/217029Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>