This will make iterating over an internal set of dirty bits simpler.

This also reorganizes the framebuffer invalidation logic a  bit,
including how the sample count is pulled from the Framebuffer.

BUG=angleproject:1156

Change-Id: I79dbcd11704ab52568c587d836af9c1bff1d2d1f
Reviewed-on: https://chromium-review.googlesource.com/529708
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>