video-converter: Implement multi-threaded scaling/conversion

video-converter: Implement multi-threaded scaling/conversion

This adds a property to select the maximum number of threads to use for
conversion and scaling. During processing, each plane is split into
an equal number of consecutive lines that are then processed by each
thread.

During tests, this gave up to 1.8x speedup with 2 threads and up to 3.2x
speedup with 4 threads when converting e.g. 1080p to 4k in v210.

https://bugzilla.gnome.org/show_bug.cgi?id=778974