Minor change in sample mask behavior for the case of sample shading

[petermcneeleychromium]

This is to make us aligned with existing spec and vulkan behavior.
Webgpu issue
gpuweb/gpuweb#5457

[mwyrzykowski]

I think this is incorrect. The current definition of sample_mask:

Sample coverage mask for the current fragment. It contains a bitmask indicating which samples in this fragment are covered by the primitive being rendered.

There is nothing in the definition indicating sample_mask is per-sample, one would it expect it to be constant per-fragment.