Move tower background update code to logic functions

Otherwise it'll go really really quickly, which is not good.

desktop_version/src/Graphics.h

@@ -197,6 +197,7 @@ public:
 	void menuoffrender();
 
 	void drawtowerbackground();
+	void updatetowerbackground();
 
 	void setcol(int t);
 	void drawfinalmap();