Move the texture index to attachment base. This will let us squash the attachment types in a follow-up patch. BUG=angleproject:963 Change-Id: I1efb2e41aa08766189499995b3150aec6fd61c4e Reviewed-on: https://chromium-review.googlesource.com/263486 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>