Force all FluidSynth sounds off when changing soundfonts

Selecting soundfonts should only happen when the MIDI engine is stopped or
reset. When stopped some notes may be left in a release phase, but we need to
ensure all fluid_sample_t objects are finished before unloading since the
soundfont (and associated sample memory) may be deleted at any time afterward.

Alc/midi/fluidsynth.c

@@ -447,6 +447,11 @@ static ALenum FSynth_selectSoundfonts(FSynth *self, ALCdevice *device, ALsizei c
     ret = MidiSynth_selectSoundfonts(STATIC_CAST(MidiSynth, self), device, count, ids);
     if(ret != AL_NO_ERROR) return ret;
 
+    ALCdevice_Lock(device);
+    for(i = 0;i < 16;i++)
+        fluid_synth_all_sounds_off(self->Synth, i);
+    ALCdevice_Unlock(device);
+
     fontid = malloc(count * sizeof(fontid[0]));
     if(fontid)
     {
@@ -695,4 +700,4 @@ MidiSynth *FSynth_create(ALCdevice* UNUSED(device))
     return NULL;
 }
 
-#endif
+#endif