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
• [ ] add support for AV1 film grain metadata