This allows issuing draw calls which only manipulate depth/stencil.

Bug: angleproject:3241
Change-Id: I62ab18a185ea5b234d3559f30c5b2b8ecb317bbb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1550900
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>