Making the GPU counter a vblank counter, not a timer.

src/xenia/gpu/graphics_system.h

@@ -52,6 +52,7 @@ public:
   virtual uint64_t ReadRegister(uint32_t r);
   virtual void WriteRegister(uint32_t r, uint64_t value);
 
+  void MarkVblank();
   void DispatchInterruptCallback(uint32_t source, uint32_t cpu = 0xFFFFFFFF);
   bool swap_pending() const { return swap_pending_; }
   void set_swap_pending(bool value) { swap_pending_ = value; }