feat(tracker): compile with midi support only when CGO is available

Also add the midi context to the VSTI, so VSTI can use MIDI if they
wish so.

tracker/model.go

@@ -117,8 +117,11 @@ type (
 		InputDevices(yield func(MIDIDevice) bool)
 		Close()
 		HasDeviceOpen() bool
+		TryToOpenBy(name string, first bool)
 	}
 
+	NullMIDIContext struct{}
+
 	MIDIDevice interface {
 		String() string
 		Open() error
@@ -385,6 +388,11 @@ func (d *modelData) Copy() modelData {
 	return ret
 }
 
+func (m NullMIDIContext) InputDevices(yield func(MIDIDevice) bool) {}
+func (m NullMIDIContext) Close()                                   {}
+func (m NullMIDIContext) HasDeviceOpen() bool                      { return false }
+func (m NullMIDIContext) TryToOpenBy(name string, first bool)      {}
+
 func (m *Model) resetSong() {
 	m.d.Song = defaultSong.Copy()
 	for _, instr := range m.d.Song.Patch {