This structure can share GL-level properties (immutably) with the
implementation.

BUG=angleproject:1369

Change-Id: I1e9406f18b6b88bb7db2a8f87b5e6d547cc7ecb1
Reviewed-on: https://chromium-review.googlesource.com/342061Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>