This prevents us from re-allocating FBO attachments every set.
This change requires quite a bit of refactoring.

BUG=angleproject:963

Change-Id: Iafa4e4a0f3dd66c9e7452e0e96a0cbb9753487bb
Reviewed-on: https://chromium-review.googlesource.com/263489Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>