Vulkan: Clean up ClipRectToScissor. Refactoring change only. Split off to make the main CL for deferring clears simpler. Bug: angleproject:4517 Change-Id: I5728a7dcf09de8952d5810061d1d8b135b625339 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2165630 Commit-Queue: Jamie Madill <jmadill@chromium.org> Commit-Queue: Tim Van Patten <timvp@google.com> Reviewed-by: Tim Van Patten <timvp@google.com>