video-converter: switch to using a thread pool ..
and expose API to allow creating a converter with a custom thread pool, and make use of that API in videoaggregator.
When setting certain properties, such as cropping or the scaled size of pads, a new converter is created by videoaggregator.
Before that patch, this implied spawning new threads, potentially at each aggregate cycle when interpolating pad properties. This is obviously wasteful, and re-using a thread pool removes that overhead.
Depends on gstreamer!682 (closed)