webgpu: speed up sample_texture_combos

[haoxli]

Render all texels in a single draw call using per-vertex point positioning instead of numAcross*2 setViewport + draw calls, and replace the O(N) select-chain sampling with an O(1) switch so each invocation samples only its own combo. Reduces runtime from ~13s to ~1s.

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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.)
• 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.
• Test descriptions are accurate and complete.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Tests avoid over-parameterization (see case count report).

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

[github-actions]

Results for build job (at 0d3d15c):

 Test case/subcase counts did not change.

[greggman]

just the formatting needs fixing

[haoxli]

Thanks for your review.