Split the validation of FBO attachment binding to common methods
and separate logic for layers and for 2D attachments.

BUG=angle:571

Change-Id: I9a59055cca60dd853311751be355440ad1e5ead0
Reviewed-on: https://chromium-review.googlesource.com/205605Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>