Emit Signal when MCSS Threads are exhausted

By default Windows has limit of 32 MMCSS threads.
Ardour uses MMCSS scheduling for for hardware I/O (audio and MIDI)
as well as for realtime process threads, and a user on a
recent machine may run into this limit.

libs/pbd/pbd/windows_mmcss.h

@@ -24,6 +24,7 @@
 
 #include <string>
 
+#include "pbd/signals.h"
 #include "pbd/libpbd_visibility.h"
 
 namespace PBD {
@@ -55,6 +56,8 @@ bool LIBPBD_API revert_thread_characteristics (HANDLE task_handle);
 
 bool LIBPBD_API set_thread_priority (HANDLE, AVRT_PRIORITY);
 
+extern Signal<void()> MMCSSError;
+
 } // namespace MMCSS
 
 } // namespace PBD

libs/pbd/windows_mmcss.cc

@@ -41,6 +41,8 @@ namespace PBD {
 
 namespace MMCSS {
 
+Signal<void()> MMCSSError;
+
 bool
 initialize ()
 {
@@ -133,6 +135,8 @@ set_thread_characteristics (const std::string& task_name, HANDLE* task_handle)
 			DEBUG_THREADS("MMCSS: Privilege not held\n");
 			break;
 		default:
+			/* this is likely because the maximum number of MMCSS threads has been reached */
+			MMCSSError (); /* EMIT SIGNAL */
 			DEBUG_THREADS("MMCSS: Unknown error setting thread characteristics\n");
 			break;
 		}