Fix scaling issue in canvas image rendering test

[greggman]

IIUC the issue is scaling can happen edge cases, especially given some APIs render with flipped Y so in the pixelated/crisp-edges case they might choose a different edge than the canvas API.

Making the display size exactly double the rendered size seems like it might solve this issue. The test would still show a difference between image-rendering: initial and image-rendeirng: pixelated / crisp-edges but should probably not show any difference otherwise

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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.

[kainino0x]

(For any other readers more info can be found on https://crbug.com/1515185)

This looks like it'll at least make the test reliable for devices with devicePixelRatio=1. For higher non-integer devicePixelRatio values I suspect we may still see the same issues though maybe this will still be more reliable on average.