Merge drawtowerbackgroundsolo() into drawtowerbackground()

It's less code being copied and pasted, especially since for my
over-30-FPS patch I would have to make a separate function for each if
both of them were still there, but if they're unified into one then I
will only have to make one more function.

And since map.scrolldir is now used outside of GAMEMODE, we'll need to
reset it in hardreset() and when exiting playtesting.

desktop_version/src/Graphics.h

@@ -185,8 +185,6 @@ public:
 
 	bool onscreen(int t);
 
-	void drawtowerbackgroundsolo();
-
 
 	void menuoffrender();