fix midi bug that caused the synth to continue to play notes after an interface stops output

src/synth/MidiController.cpp

@@ -88,6 +88,18 @@ void MidiController::handleMessage(const std::vector<unsigned char>& msg) {
     if(status == 0xFE) return; // "Active Sensing" -> 300ms heartbeat. could be useful to sense if this is missing for device failure detection
     if(status == 0xF8) return; // "Timing Clock" -> 24 pulses per quarter note, for steady rhythm. not useful for this instrument
 
+    if(status == 0xB0) { // channel mode change
+        if((data1 & 0xF0) == 0x70) { // all notes off for this channel
+            for(uint8_t i = 0; i < UINT8_MAX; i++) {
+                noteOff(i);
+            }
+            return;
+        }
+        // TODO: msg[0] contains the channel to turn all notes off for
+        // since the current implementation is channel agnostic, we just turn all notes off
+        // eventually we might have noteOff(channelLookup(msg[0]), i);
+    }
+
     // sustain pedal message event
     if(status == 0xB0 && data1 == 64) {
         handleSustain(data2 >= 64);