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>
