Document colorimetry aspects of video decoding, fix mixing up primaries & matrix coefficients

[Wumpf]

What

Learned a lot more stuff recently which is now documented better.
Previously incorrectly called things out as color primaries when actually the difference is not the primaries (they are the same unless you go hairsplitting with PAL & NTSC), but the "Matrix Coefficients"

There's a small functional change in this pr - use dav1d's matrix_coefficients instead of color_primaries - but since we're very likely to still almost always choose Bt709 coefficients I expect this to be a very very minor change in practice.

Meaning to say the meat of this is the docs & renames ;-)

Checklist

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  • Using examples from latest main build: rerun.io/viewer
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• If have noted any breaking changes to the log API in CHANGELOG.md and the migration guide

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[emilk]

This is great 🙏

[Wumpf]

or horrible, depending how you look at it 😄