Vulkan: Implement masked color clears. This implements masked color clear using clear shaders. The shaders themselves were introduced in a prior patch. In order to get the right setup for the draw call to trigger the shaders, we create an internal pipeline from the pipeline cache. We also use a special pipeline layout with only uniform buffers. The masked out color channels are disabled via settings on the pipeline. This fixes the dEQP masked color clear tests. It doesn't handle masked color clears combined with the depth clear bit. It's likely we don't have test coverage for this case. Bug: angleproject:2455 Change-Id: I513248cc0f7e58f490fc16ac9afb40119d730ccc Reviewed-on: https://chromium-review.googlesource.com/1031373 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>