[graphite] Add wgpu R16Unorm and RG16Unorm

The formats are needed to import HDR YUV textures.

Bug: chromium:1471362
Change-Id: I33c88d5c907aa7a8b193cf0bc9776f5634f1a633
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/742636
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
4 files changed