Vulkan: Clean up ClipRectToScissor.
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 <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>
Commit-Queue: Tim Van Patten <c7814369a8d3d5671c484acc7b3215f1bf4221be@google.com>
Reviewed-by: Tim Van Patten <c7814369a8d3d5671c484acc7b3215f1bf4221be@google.com>

ppy/angle,ppy/angle,ppy/angle,ppy/angle