Synchronize DVDInterface::ChangeDisc with the CPU thread properly.

This addresses a bit of thread unsafety mentioned in a comment, and
fixes a 'ScheduleEvent_Threadsafe from main thread' message.

To make this work nicely, make PauseAndLock call DeclareAsCPUThread -
i.e. while you have the CPU thread locked, you can consider yourself the
CPU thread.

Source/Core/Core/Core.h

@@ -41,6 +41,9 @@ bool Init();
 void Stop();
 void Shutdown();
 
+void DeclareAsCPUThread();
+void UndeclareAsCPUThread();
+
 std::string StopMessage(bool, const std::string&);
 
 bool IsRunning();