MIDI - flush unsent MIDI messages on Run Stop

If MIDI messages are sent too quickly then they can get buffered in the sp_midi nif. This will flush these unsent and buffered messages so that they don't keep being sent as the buffer naturally empties.

app/server/ruby/lib/sonicpi/runtime.rb

@@ -397,11 +397,18 @@ def __stop_jobs
       # Flush OSC messages on Erlang scheduler
       __osc_flush!
 
+      # Flush MIDI messages within sp_midi nif
+      __midi_flush!
+
       # Force a GC collection now everything has stopped
       GC.start
       GC.compact if RUBY_VERSION >= "2.7.0"
     end
 
+    def __midi_flush!
+      @osc_client.send("/midi_flush")
+    end
+
     def __midi_system_reset(silent=false)
       __info "Resetting MIDI subsystems..." unless silent
       __schedule_delayed_blocks_and_messages!