BUG=angleproject:2306

+ common validation for multitexturing units
+ clang-format

Change-Id: I6eb456c273490e85fc7008e7e11d15e22dd20276
Reviewed-on: https://chromium-review.googlesource.com/987298
Commit-Queue: Lingfeng Yang <lfy@google.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>