Split VideoBackend::Cleanup from Shutdown.

First is called from ogl/d3d thread, second is called from emulation thread (x11...)

Source/Core/Common/Src/VideoBackendBase.h

@@ -102,6 +102,7 @@ public:
 	virtual void Video_Prepare() = 0;
 	virtual void Video_EnterLoop() = 0;
 	virtual void Video_ExitLoop() = 0;
+	virtual void Video_Cleanup() = 0; // called from gl/d3d thread
 
 	virtual void Video_BeginField(u32, FieldType, u32, u32) = 0;
 	virtual void Video_EndField() = 0;