Fix a minor issue introduced in previous stencil test fix

[toji]

I flubbed one line in #3083, not realizing that the row stride needed to be computed differently for the source data and readback data. Previous code worked fine for the offsets_and_sizes_copy_depth_stencil tests, but failed for the rowsPerImage_and_bytesPerRow_depth_stencil tests. This fix allows both to work as expected.

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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.

[github-actions]

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