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>