fftools/ffmpeg: convert to a threaded architecture

Change the main loop and every component (demuxers, decoders, filters,
encoders, muxers) to use the previously added transcode scheduler. Every
instance of every such component was already running in a separate
thread, but now they can actually run in parallel.

Changes the results of ffmpeg-fix_sub_duration_heartbeat - tested by
JEEB to be more correct and deterministic.

Changelog

@@ -7,6 +7,8 @@ version <next>:
 - EVC encoding using external library libxeve
 - QOA decoder and demuxer
 - aap filter
+- demuxing, decoding, filtering, encoding, and muxing in the
+  ffmpeg CLI now all run in parallel
 
 version 6.1:
 - libaribcaption decoder