pulseaudio: Move to a threaded mainloop.

This used to create a context and mainloop for each device and the hotplug
thread, but this isn't correct use of PulseAudio's API. Now we have a
single context and a pa_threaded_mainloop, and all threads cooperate around
it.

This was originally from SDL3, in 35292d7dba.

Reference Issue #7883.
Reference Issue #7427.

src/audio/pulseaudio/SDL_pulseaudio.h

@@ -35,8 +35,6 @@ struct SDL_PrivateAudioData
     char *device_name;
 
     /* pulseaudio structures */
-    pa_mainloop *mainloop;
-    pa_context *context;
     pa_stream *stream;
 
     /* Raw mixing buffer */