Ensure oldcutscenebars is updated when cutscenebarspos is

To do this, I've added Graphics::setbars(), to make sure
oldcutscenebarspos always gets assigned when cutscenebarspos is. This
fixes potential deltaframe rendering issues if these two mismatch.

desktop_version/src/Graphics.h

@@ -83,6 +83,7 @@ public:
 
 	void cutscenebars(void);
 	void cutscenebarstimer(void);
+	void setbars(const int position);
 
 	void drawpartimage(int t, int xp, int yp, int wp, int hp);