backend-pipewire: add PipeWire backend
Add a separate PipeWire backend based on the PipeWire plugin. The backend requires PipeWire 0.3.x.
The PipeWire backend can be used as a standalone-backend backend for streaming and composing Wayland clients to PipeWire.
The backend supports the on-demand creation of heads via the weston_pipewire_output_api_v1. It also supports per-output pixel format configuration via a gbm-format option.
Multiple PipeWire outputs can be created by setting the num-outputs option in the [pipewire] section.