Missing multisample state validation

[andyleiserson]

There are failures in the CTS suite webgpu:api,validation,render_pipeline,multisample_state:*, possibly attributable to the following validation items:

https://gpuweb.github.io/gpuweb/#abstract-opdef-validating-gpumultisamplestate:

• If descriptor.alphaToCoverageEnabled is true:
  • descriptor.count > 1.

https://gpuweb.github.io/gpuweb/#abstract-opdef-validating-gpurenderpipelinedescriptor:

• If descriptor.fragment is provided:
  • If the sample_mask builtin is a shader stage output of descriptor.fragment:
    • descriptor.multisample.alphaToCoverageEnabled is false.