Added 16-bit formats to GLConfigs to reduce offscreen texture memory usage. BUG=angleproject:1042 Change-Id: I7024bd47601a21b08cafbf6460d512151b53d035 Reviewed-on: https://chromium-review.googlesource.com/277002 Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>