GLES1: Texture environments setup - Revise entry point definitions to use packed enums BUG=angleproject:2306 Change-Id: I06ad95f475d1dbaf07ec24ff2544503c4a44e826 Reviewed-on: https://chromium-review.googlesource.com/1090996 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Lingfeng Yang <lfy@google.com>