ES31: Add support for bindImageTexture on GL backend This patch refers to https://chromium-review.googlesource.com/c/380636/ BUG=angleproject:1987 Change-Id: If621eed6ecaa7298214843a2a133801ca1487b03 Reviewed-on: https://chromium-review.googlesource.com/462088 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>