Since FBO attachments and Renderbuffers are semantically distinct,
make a new location for attachments to live.

BUG=angle:660

Change-Id: I51753f8a814e89641637c5d8293f7e9a573a7ba5
Reviewed-on: https://chromium-review.googlesource.com/201833Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>