demux: fix a comment

Don't refer to fields that were removed.
vf_vapoursynth: resolve paths relative to home/config

This affects the script filename passed to the filter. Resolve "~" (and
some other variants) as described in the "Paths" section of mpv.rst.
vf_vapoursynth: don't error if invoke() doesn't return a clip

Not all functions are for creating filters. Consider for example
LoadPlugin.