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 ! ...

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
2 jobs for pipewire in 1 minute and 21 seconds (queued for 1 second)
Status Job ID Name Coverage
  Container Prep
passed #432170
container_prep

00:00:21

 
  Build
passed #432171
build-native-meson

00:01:00