Optimize ValidateDrawAttribs: Part 3. This is a small optimization for the WebGL compatibility mode. Instead of scanning the list of attributes for a Transform feedback conflict, it can quickly check a cached mask. This should save a lot of cycles on the fast path. Bug: angleproject:1391 Change-Id: Icb8d095493a629dbff0e93872357e5bf7c7458ae Reviewed-on: https://chromium-review.googlesource.com/1011236 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org>