Mark GL_RGB10_A2UI as a renderable format. BUG=angleproject:1149 Change-Id: Ia808c0d411133d135eef8e90485ba346086f012c Reviewed-on: https://chromium-review.googlesource.com/296964 Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org> Tested-by: Geoff Lang <geofflang@chromium.org>