video-converter: switch to using a task pool ..

video-converter: switch to using a task 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 task pool removes that
overhead.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/896>