Similarly to the D24S8 attachment getting confused when we use only
the stencil portion, ANGLE could write to the stencil portion when
we were only using the depth. Fix this by internally disabling stencil
test and writes when using this type of configuration.

BUG=angleproject:1237

Change-Id: Ib7868d5e9f8aea73304a132005541dab947aa619
Reviewed-on: https://chromium-review.googlesource.com/314032
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>