Methods that need to mutate the Framebuffer should use
setAttachment, etc, instead of using mutable pointers.

BUG=angleproject:963

Change-Id: Ibe7b8f8245f762916e0224fdc78796b1c933195f
Reviewed-on: https://chromium-review.googlesource.com/263490Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>