Dawn: work around a fragcoord.w issue on D3D12.

D3D12 supplies w in fragcoord.w, while other backends supply 1/w.
This may be better handled in Dawn or SPIRV-Cross, but for now we'll work around the issue here.

Bug: skia:10475
Change-Id: I8f32f71f8f6b454365fbd0728fe063de2c150b8a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/300711
Commit-Queue: Stephen White <senorblanco@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
1 file changed