Amends CopyTexImage validation to conform to new language in the ES 3.0.3 spec. bug=angle:582 Also performs format checks for non-subimage copies, and permits unsized format textures as destination for the copy. Change-Id: Id647bb968416fbd35e1c70e5da21c81367e08f07 Reviewed-on: https://chromium-review.googlesource.com/189431 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Shannon Woods <shannonwoods@chromium.org>