Make the filter apply the pixel aspect ratio of the input to the output.
This is more useful than forcing 1:1 PAR when playing anamorphic video
such as DVDs.
VapourSynth itself actually allows passing through the aspect ratio, but
it's in a not very useful form for us: it's per video-frame instead of
constant (i.e in VSVideoInfo). As long as we don't have a way to allow a
filter to spontaneously change output parameters, we can't use this.
(And I don't really feel like making this possible.)
We were relying on vsscript_freeScript() to take care of proper
termination. But it doesn't do that: it doesn't wait for the filters to
finish and exit at all. Instead, it just destroys all objects, which
causes the worker threads to crash sometimes.
Also, we're supposed to wait for the frame callback to finish before
freeing the associated node.
Handle this by explicitly waiting as far as we can. Probably fixes
crashes on seeking, although VapourSynth itself might also need some
work to make this case completely stable.
Before this commit, the filter attempted to keep the vsscript state
(p->se) even when the script was reloaded. Change it to destroy the
script state too on reloading. Now no workaround for LoadPlugin is
necessary, and this also fixes a weird theoretical race condition when
destroying and recreating the mpv source filter.
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.