Encoding: Add an environment variable to force encoder type

This adds an environment variable, KPIPEWIRE_FORCE_ENCODER, that, when set to the right value, will force using a certain encoder. Most importantly, this allows forcing the use of libx264 software encoding for H264 if hardware encoding fails for some reason. It also allows forcing hardware encoding if for some reason aborting when it fails is preferred.

This is mostly intended as a debugging help.