Renderbuffers are a clear object type in GL, and this patch adds
a more consistent state representation for them. They're managed
by the ResourceManager, and have a storage implementation similar
to Textures, but much simpler.

BUG=angle:660

Change-Id: Ia17199bb8cb570d48db42e1f28ccbcc12a902fcf
Reviewed-on: https://chromium-review.googlesource.com/201834Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>