Add pipewire plugin

It is quite similar to the remoting plugin. It just exports the frames via pipewire instead of the builtin GStreamer pipeline. It implements the same virtual output API. Virtual outputs can be created by adding 'pipewire-output' sections to weston.ini.

The generated frames can be accessed with any pipewire client. e.g. with GStreamer:

gst-launch-1.0 pipewiresrc ! video/x-raw,format=BGRx ! ...