op,render_pipeline,alpha_to_coverage

[shrekshao]

Issue: #2201

Add the alphaToCoverage mask tests.

Remove the src/webgpu/api/operation/render_pipeline/alpha_to_coverage.spec.ts stub and put tests under src/webgpu/api/operation/render_pipeline/sample_mask.spec.ts to share code.

The algorithm of producing the alpha-to-coverage mask is platform-dependent. And the requirement of alphaToCoverage mask of alpha where 0 < alpha < 1 is only that the mask bits should be non-decreasing.
So instead of putting alpha = { < 0, = 0, = 1/16, = 2/16, ..., = 15/16, = 1, > 1 } in params subcases, put them inside the test. Draw and readback the value for each alpha value. Iterate through them and compare the result value with that of the previous alpha value (alpha'). Check if the result is non-decreasing for each component (for color, stencil), or non-increasing if the value is in negative-correlation to the alpha mask (depth)

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

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[kainino0x]

LGTM with minor comments

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