Add support for the pixel_local modifier to WGSL.

This keyword isn't fully supported by WGSL yet, so it generates
an error when compiled.

https://docs.google.com/document/d/1djJwQLJcVGnDXOA7nhsweppim-hhbqglB4JNyfZcRqQ

Bug: b/299504320
Bug: dawn:1704
Change-Id: Ifb1fcc0192d6aa0087e93de12c269eb38251ff7b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/753566
Reviewed-by: Arman Uguray <armansito@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
7 files changed