Add test for scissored clear. This simple test will be used to debug the Vulkan back-end's basic scissored clear functionality. Bug: angleproject:2356 Change-Id: I4bed68740ac57b3cc61f2dfe9164081b05bc31b3 Reviewed-on: https://chromium-review.googlesource.com/914230 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>