ao_pipewire: Add PipeWire audio backend

The AO provides a way for mpv to directly submit audio to the PipeWire
audio server.
Doing this directly instead of going through the various compatibility
layers provided by PipeWire has the following advantages:

* It reduces complexity of going through the compatibility layers
* It allows a richer integration between mpv and PipeWire
  (for example for metadata)
* Some users report issues with the compatibility layers that to not
  occur with the native AO

For now the AO is ordered after all the other relevant AOs, so it will
most probably not be picked up by default.
This is for the following reasons:

* Currently it is not possible to detect if the PipeWire daemon that mpv
  connects to is actually driving the system audio.
  (https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/1835)
* It gives the AO time to stabilize before it is used by everyone.

Based-on-patch-by: Oschowa <oschowa@web.de>
Based-on-patch-by: Andreas Kempf <aakempf@gmail.com>
Helped-by: Ivan <etircopyhdot@gmail.com>

DOCS/man/ao.rst

@@ -144,6 +144,17 @@ Available audio output drivers are:
         Allow mpv to use PulseAudio even if the sink is suspended (default: no).
         Can be useful if PulseAudio is running as a bridge to jack and mpv has its sink-input set to the one jack is using.
 
+``pipewire``
+    PipeWire audio output driver
+
+    The following global options are supported by this audio output:
+
+    ``--pipewire-buffer=<1-2000|native>``
+        Set the audio buffer size in milliseconds. A higher value buffers
+        more data, and has a lower probability of buffer underruns. A smaller
+        value makes the audio stream react faster, e.g. to playback speed
+        changes.
+
 ``sdl``
     SDL 1.2+ audio output driver. Should work on any platform supported by SDL
     1.2, but may require the ``SDL_AUDIODRIVER`` environment variable to be set

audio/out/ao.c

@@ -41,6 +41,7 @@ extern const struct ao_driver audio_out_audiounit;
 extern const struct ao_driver audio_out_coreaudio;
 extern const struct ao_driver audio_out_coreaudio_exclusive;
 extern const struct ao_driver audio_out_rsound;
+extern const struct ao_driver audio_out_pipewire;
 extern const struct ao_driver audio_out_pulse;
 extern const struct ao_driver audio_out_jack;
 extern const struct ao_driver audio_out_openal;
@@ -87,6 +88,9 @@ static const struct ao_driver * const audio_out_drivers[] = {
 #endif
 #if HAVE_SDL2_AUDIO
     &audio_out_sdl,
+#endif
+#if HAVE_PIPEWIRE
+    &audio_out_pipewire,
 #endif
     &audio_out_null,
 #if HAVE_COREAUDIO