Add utils.detectVideoAlphaMode

[dev7355608]

Description of change

As you might know, for no good reason, under some circumstances video alpha is already premultiplied by whatever is responsible for uploading the pixels to the GPU. My suspicion is that it is a bug in ANGLE. If I set the ANGLE backend is set to OpenGL, then the video is uploaded as expected: unless the alpha mode is UNPACK, the video is not premultiplied on upload. If the ANGLE backend is set to D3D11 in Chrome, then (on my machine) the video is already premultiplied: so, if the alpha mode is UNPACK, if is premultiplied twice (but there is another bug: if the video dimensions are 1x1 pixels, then no extra premultiplication is applied in Chrome; in Firefox with the D3D11 backend 1x1 behaves like any other dimension). To workaround this issue I wrote a function that checks whether a video is premultiplied without premultiply on upload, in which case the alpha mode PMA is returned instead of UNPACK.

This has been a widespread issue for years. But I can't find a bug report for this in the Chrome and ANGLE issue tracker. I think, it might go unnoticed a lot of the times: the developers/artist machine might render it correctly and the user doesn't know what the asset is supposed to look like. For a long time I thought it the assets might have been generated with premultiplied alpha by mistake; that's also what the author of #7836 thought, but the asset is actually encoded with straight alpha as it should be.

I'd like to combine this with #9314, so that the video loader uses utils.detectVideoAlphaMode() by default if alphaMode isn't specifically requested.

• Before: https://www.pixiplayground.com/#/edit/vhlX3qbIqAWaGvA_cxFZ1
• After: https://www.pixiplayground.com/#/edit/CBwvohPYOWqjoXyr4BvCS

1. View test in Chrome.
2. Go to chrome://flags.
3. Search for Choose ANGLE graphics backend.
4. Switch between D3D11 and OpenGL.

Pre-Merge Checklist

• Tests and/or benchmarks are included
• Documentation is changed or added
• Lint process passed (npm run lint)
• Tests passed (npm run test)

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[GoodBoyDigital]

nice!