This commit also adds tests for clearing depth and stencil with
mask/scissor.

Bug: angleproject:2540
Change-Id: I30dd840afd6cdd4e3a38c50fcba4c8623513ceb0
Reviewed-on: https://chromium-review.googlesource.com/c/1307585Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>