snes: MSU1 audio support by dent^nz and ElectronAsh (#602)

Co-authored-by: Karl Lurman <karl.lurman@wilsongroupau.com>

user_io.cpp

@@ -2573,6 +2573,13 @@ int user_io_file_tx(const char* name, unsigned char index, char opensave, char m
 	}
 
 	ProgressMessage(0, 0, 0, 0);
+
+    if (is_snes())
+    {
+        // Setup MSU
+        snes_msu_init(name);
+    }
+
 	return 1;
 }
 
@@ -2819,6 +2826,7 @@ void user_io_poll()
 	else if ((core_type == CORE_TYPE_8BIT) && !is_menu() && !is_minimig())
 	{
 		if (is_st()) tos_poll();
+		if (is_snes()) snes_poll();
 
 		for (int i = 0; i < 4; i++)
 		{