*re-land with fix for Mac*

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

BUG=angleproject:963

Change-Id: Ia1f83e3c427d446ddbe16c6703db136942149e91
Reviewed-on: https://chromium-review.googlesource.com/266691Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>