BUG=angle:782

Also eliminates Colorbuffer/DepthStencilBuffer and related friends
and adds the RenderbufferD3D implementation of RenderbufferImpl.

Change-Id: Ibe72c9589b3c238b952744e7cfd70d97ec898eb9
Reviewed-on: https://chromium-review.googlesource.com/222925Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Shannon Woods <shannonwoods@chromium.org>