Remove queue of pending frames in the encoder

Currently kpipewire's h264 encoder has a queue of pending frames. If a frame comes in, nothing comes out until another frame goes in. This leaves us always with at least 1 frame of latency.

As we only get frames when things happen that 1 frame of latency could happen minutes later. This is mitigated by arguably hacky code in pipewirerecord to replay existing frames, but it's still always a delay.

There is a tuning flag that gets rid of this delay and allows us to drop the previous hack.