When using STENCIL8, we emulate it on D3D11 with D24S8, since there
is no native stencil-only format. However in many places we would
write to the depth part of this format, and confuse the D3D runtime
when it would use the depth test. Fix this by never modifying the
depth portion of the buffer, or reading from it.

BUG=angleproject:1232

Change-Id: Ifd2e54eceae84e8deea85f439c132d07981b2286
Reviewed-on: https://chromium-review.googlesource.com/313996
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>