pulseaudio: wait until at least 1/8th (!) of the mixbuffer is available.

This is to workaround systems where we hang in playback because the buffer
does not report the space for whatever reason. The system will instead block
in PlayDevice, which always immediately follows WaitDevice in modern times
so this works out, and it seems to keep the device moving forward.

For a future revision, we are either going to clean this up more properly,
or attempt to move to PulseAudio's pa_stream_set_write_callback() API, but
this will do for SDL 2.0.18.

Reference #4387 for discussion and further information.

src/audio/pulseaudio/SDL_pulseaudio.c

@@ -368,7 +368,7 @@ PULSEAUDIO_WaitDevice(_THIS)
             SDL_OpenedAudioDeviceDisconnected(this);
             return;
         }
-        if (PULSEAUDIO_pa_stream_writable_size(h->stream) >= h->mixlen) {
+        if (PULSEAUDIO_pa_stream_writable_size(h->stream) >= (h->mixlen/8)) {
             return;
         }
     }