mpv: Move all wrappings to a single wrapper Nix function

Inspired by `wrapNeovim`, write a wrapMpv Nix function that creates a
derivation that has all of the environment that was added if needed at
the unwrapped version.

Add derivations to all-packages.nix in an almost compatible way and make
`mpv-with-scripts` throw a message implying to switch to `wrapMpv` which
has an incompatible signature.

Add to vapoursynth a new passthru attribute `python3` that is used in
passed down to the wrapper to ensure ABI compatibility with
`PYTHONPATH`.