maxInterStageShaderVariables: decrement max fragment input outputs per fragment input built-in

[ErichDonGubler]

Issue: #4538

Recommended review experience is commit-by-commit.

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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 (see maxInterStageShaderVariables should be decremented for _each_ fragment input built-in, not just once #4538 ).
• Test have be tested with compatibility mode validation enabled and behave as expected. (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.

[dneto0]

Nice catch. LGTM.

This might explain some failures in Chrome/Dawn too.

• crbug.com/407159803
• crbug.com/417770938

[dneto0]

FYI: Run npm run fix to reformat the code

[ErichDonGubler]

NOTE: Per @kainino0x's question about coverage (see #4538 (comment)), I'm adding more coverage for the rest of the fragment input built-ins with d00cd2c. @dneto0: The re-review should be fast! 🙏🏻

[ErichDonGubler]

Hmm…with the CI failures, I'm not sure how to handle this immediately. I'd rather lock in the reviewed-and-passing stuff I initially added, and break out the fix for gpuweb/gpuweb#5403 separately. I've rolled back to 5587128, and will file a separate issue/PR pair.