This special format means we're using a depth/stencil format with only
the stencil channel being used. We can just use a normal D24S8 format
here, there's pretty much no difference.

BUG=angleproject:1455

Change-Id: Id304094b92814f38930209a703733190ad8e8410
Reviewed-on: https://chromium-review.googlesource.com/359086Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>