Add glsl-shader-opts to vf_libplacebo

[Aaron]

Summary of the bug:
Liblacebo has a key=value option "glsl-shader-opts" to use for tunable parameters in custom shaders:
​https://mpv.io/manual/master/#options-glsl-shader-opts
Can it be added to vf_libplacebo?
vf_libplacebo has "extra_opts":
https://ffmpeg.org/ffmpeg-filters.html#toc-Output-mode
But it's only for key=value pairs
How to reproduce:

% ffmpeg -i input -vf "libplacebo=extra_opts='glsl-shader-opts=contrast=1'" output