cameraCapture: Simplify codepaths, try to wait for a real frame

[kainino0x]

• Don't use fallback paths in camera capture; instead, have the test pick the exact path it's using, and parameterize over it. Test two paths: VideoFrame from MediaStreamTrackProcessor, and the old-fashioned HTMLVideoElement.
• Work around an issue where Chrome (at least on Mac) shows blank frames for a while after initializing the camera.
• I happened to notice that requesting different width/height from the camera is broken in Chrome in several ways, so added cases for that.
• Added a TODO for copyExternalImageToTexture from camera

These tests only worked in Chrome, and even then they were buggy (usually running the test on a blank first frame). Now, the HTMLVideoElement tests work in Safari.

Test failures are as follows, on an M1 Mac:

• Chrome: https://crbug.com/411656657
  • HTMLVideoElement passes some cases, but fails when a requested size is passed to getUserMedia().
  • VideoFrame fails due to incorrect color management (regardless of dstColorSpace).
• Safari:
  • HTMLVideoElement fails due to incorrect color management (regardless of dstColorSpace).
  • VideoFrame is skipped.
• Firefox: doesn't yet implement importExternalTexture.

Issue: fixes #4363, cc #4364

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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 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.

[shaoboyan091]

Thanks for fixing! Some questions.

[kainino0x]

Oops, wait, I still haven't fixed the other code path.

[kainino0x]

Done, @shaoboyan091 PTAL. Thanks for keeping me honest in using proper logic instead of timeout hacks.

[shaoboyan091]

Looks great!