Vulkan: Add immediate scissored clears. This allows us to avoid using draw commands with pipelines for scissored clears. This prevents some tests from generating large numbers of VkPipelines. Bug: angleproject:4517 Bug: angleproject:4617 Change-Id: Id4a44000078098a60aa89233cfef30b75727d108 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2194473 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com>