BlitGL: add a ScopedGLState to separate our state from the app's BUG=angleproject:1356 Change-Id: I19ec4efa37d142ac7370815b078f4ba79caa0bea Reviewed-on: https://chromium-review.googlesource.com/412451 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>