Implement multisample "disable" in Vulkan

Emulates multisample disable by colocating all samples at pixel
center. Multisample disable will be required in order for us to
integrate mixed samples.

Change-Id: I52b67e22c979584b5d001e737a08eef07bd7ffe2
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253282
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
7 files changed