Add encoder using libopenh264
On some distributions, libopenh264 is the only encoder available OOTB. Add support for it and use it as fallback.
Needed a rewrite of how KPIPEWIRE_FORCE_ENCODER is handled, see that commit for details.
Tested using xfreerdp from freerdp 3.5.1, works. Quality isn't particularly great, but it's usable. Not sure whether that can be improved easily.
Edited by Fabian Vogt