Make Scheduler a singleton

src/backend/Core.cpp

@@ -82,7 +82,7 @@ void Core::Run(float volumeL, float volumeR) {
 
         cycles += taken;
         frameCycles += taken;
-        scheduler.Tick(taken, mem);
+        Scheduler::GetInstance().Tick(taken, mem);
       }
 
       cycles -= mmio.vi.cyclesPerHalfline;
@@ -93,7 +93,7 @@ void Core::Run(float volumeL, float volumeR) {
     }
 
     mmio.ai.Step(frameCycles, volumeL, volumeR);
-    scheduler.Tick(frameCycles, mem);
+    Scheduler::GetInstance().Tick(frameCycles, mem);
   }
 }