Mask stencil clear value and reference value with 255

[beicause]

Connections
Fix webgpu:api,operation,render_pass,clear_value:stencil_clear_value:* on metal.
Found this issue in #9568.

Description
See gpuweb/gpuweb#2921 and https://issues.chromium.org/issues/42240605, we need to take the LSBs of stencil value. Currently only 8-bit stencil formats are supported so value & 255.

Testing

cargo xtask cts `webgpu:api,operation,render_pass,clear_value:stencil_clear_value:*`

Squash or Rebase?
Squash

Checklist

• I self-reviewed and fully understand this PR.
• WebGPU implementations built with wgpu may be affected behaviorally.
• Validation and feature gates are in place to confine behavioral changes.
• Tests demonstrate the validation and altered logic works.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and doesn't make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.

[andyleiserson]

I don't actually see truncation of the reference value in the spec, but that is what the CTS wants, and seems right to me. Filed gpuweb/gpuweb#6273 for that.

[andyleiserson]

Thanks!