mpv: add script infrastructure & convert script

mpv’s functionality can be modified with lua scripts, by specifying them
on the command line.
`scripts` is a list of lua files that are appended to the default mpv
invocation. We also provide a `mpvScripts` attrset with the available
scripts in the top namespace.

`convert` is one such script, that extends mpv with a simple but very
convenient on-the-fly cropping and converting feature.

Closes #14040.