fix all 4 backends' failure to include the main "backend" thread when computing ::in_process_thread()

libs/backends/jack/jack_audiobackend.cc

@@ -862,6 +862,16 @@ JACKAudioBackend::join_process_threads ()
 bool
 JACKAudioBackend::in_process_thread ()
 {
+#ifdef COMPILER_MINGW
+	if (_main_thread == GetCurrentThread()) {
+		return true;
+	}
+#else // pthreads
+	if (pthread_equal (_main_thread, pthread_self()) != 0) {
+		return true;
+	}
+#endif
+
 	for (std::vector<jack_native_thread_t>::const_iterator i = _jack_threads.begin ();
 	     i != _jack_threads.end(); i++) {
 
@@ -909,6 +919,8 @@ JACKAudioBackend::process_thread ()
         /* JACK doesn't do this for us when we use the wait API
          */
 
+	_main_thread = pthread_self ();
+
         AudioEngine::thread_init_callback (this);
 
         while (1) {

libs/backends/jack/jack_audiobackend.h

@@ -209,6 +209,7 @@ class JACKAudioBackend : public AudioBackend {
     std::map<DataType,size_t> _raw_buffer_sizes;
 
     std::vector<jack_native_thread_t> _jack_threads;
+    jack_native_thread_t _main_thread;
 
     static int  _xrun_callback (void *arg);
     static void* _process_thread (void *arg);