video: add VapourSynth filter bridge

Mainly meant to apply simple VapourSynth filters to video at runtime.
This has various restrictions, which are listed in the manpage.

Additionally, this actually copies video frames when converting frame
references from mpv to VapourSynth, and a second time when going from
VapourSynth to mpv. This is inefficient and could probably be easily
improved. But for now, this is simpler, and in fact I'm not sure if
we even can references VapourSynth frames after the core has been
destroyed.
client API: add mpv_get_wakeup_pipe convenience function

Should make integreating with some event loops easier. Untested.
lua: wrap mpv_get_wakeup_pipe()

Pretty much experimental for issue #661.
lua: make it easier to integrate with foreign event loops

We provide some "official" utility functions for this.
manpage: lua: separate "advanced" functions into a separate section

This will be less confusing.