(WIP) utils/libav: add helpers for mangling av pixfmt into libplacebo

These structs are designed in not-quite-compatible ways, and the mapping
is not at all intuitive or obvious.

TODO:
- more testing, make sure it actually does the right thing
- more helper functions (cf. colorspace conversions etc.)
- pin down minimum version requirement?
- figure out how to handle yuv420p10 etc. properly
11 jobs for avpixfmt in 2 minutes and 31 seconds (queued for 1 second)
latest
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  Build
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debian aarch64
build-debian-aarch64

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macos
build-macos

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amd64 debian
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amd64 debian
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win32
build-win32

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passed #315398
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build-win64

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  Test
passed #315405
gpu
test-gpu

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passed #315401
amd64 debian
test-ubuntu-asan

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amd64 debian
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amd64 debian
test-ubuntu-scan

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amd64 debian
test-ubuntu-ubsan

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