avfilter/palette{gen,use}: add palette utils

These color management helpers will be shared by palettegen and
paletteuse in the following commits.

Note that it probably makes sense to share at least the sRGB/linear
functions with other filters at some point.

More information on OkLab can be found here: https://bottosson.github.io/posts/oklab/

For the arithmetic integer version, see:
http://blog.pkh.me/p/38-porting-oklab-colorspace-to-integer-arithmetic.html
and https://github.com/ubitux/oklab-int

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