Refactor common attachment validation to base method. 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/205605 Reviewed-by: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shannon Woods <shannonwoods@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>