D3D11: Don't read or write to the unused depth buffer. 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>