vf_vapoursynth: abstract scripting backend

In theory, vsscript should be doing it, but it's not there yet, neither
did there seem to be any interest in making it flexible enough to handle
more than 1 scripting language.
vf_vapoursynth: add standalone Lua scripting
gl_x11: distinguish missing GLX and too old GLX versions

This probably led to confusing output.

CC: @mpv-player/stable
DOCS/edl-mpv: minor clarification