wx doesn't seem to like the dialogs being created on some other worker
thread, which makes file dialogs opened by lua scripts crash in various
ways on Linux. Doing everything on the main thread hopefully fixes this.
Add a utility library that wraps LWLibavSource and can parse its .lwi
file to obtain timecodes and keyframes. It also contains a function to
generate and save keyframes using WWXD or Scxvid. Update the default
scripts to use these functions.
The patched meson.build for dav1d just changes the include_directories
from 'include/dav1d' to 'include' and, while we're at it, removes the
pkg_config stuff to avoid a deprecation warning.
AV1 decoding is pretty broken in upstream ffms2, so point the wrap to a
fork that has some hack patches for it. BestSource works fine out of the