We can use the flat FBO attachment types to update our data
instead of reallocating it. This also lets us delete the
DefaultFramebuffer class.

BUG=angleproject:963,angleproject:840

Change-Id: Ib8f20d8212d073fb5e248756321cb2e6b4e086dc
Reviewed-on: https://chromium-review.googlesource.com/263492Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>