This script breaks peridocially because we don't have automated tests
for it. Split the FBO attachment object class into its own file, and
also fix a couple other small snags that prevented the script from
running.

This will facilitate generating Vulkan renderer stubs.

BUG=angleproject:1319

Change-Id: I30a6ce4ab0adad962cea76731dbe82837c5c9a1b
Reviewed-on: https://chromium-review.googlesource.com/347064Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>