We can store all relevant information in the base class, which
lets us avoid using any virtual methods. This will finally let
us avoid using reallocations on FBO attachment sets.

BUG=angleproject:963

Change-Id: Ib4b61da14efaf843478b059499c01e34f9c65e4f
Reviewed-on: https://chromium-review.googlesource.com/263488Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>