output/pipewire: new output plugin

Very rough draft.  Barely works.

src/lib/pipewire/meson.build

@@ -0,0 +1,14 @@
+pipewire_dep = dependency('libpipewire-0.3', required: get_option('pipewire'))
+conf.set('ENABLE_PIPEWIRE', pipewire_dep.found())
+if not pipewire_dep.found()
+  subdir_done()
+endif
+
+pipewire_dep = declare_dependency(
+  dependencies: pipewire_dep,
+
+  # disabling -Wpedantic because libpipewire's headers are not
+  # compatible with C++; using the "#pragma" is not enough, we need to
+  # disable it at the command line
+  compile_args: ['-Wno-pedantic'],
+)