Refactor tower background to use a separate object instead

Previously, the tower background was controlled by a disparate set of
attributes on Graphics and mapclass, and wasn't really encapsulated. (If
that's what that word means, I don't particularly care about
object-oriented lingo.) But now, all relevant things that a tower
background has has been put into a TowerBG struct, so it will be easy to
make multiple copies without having to duplicate the code that handles
it.

desktop_version/src/TowerBG.h

@@ -0,0 +1,17 @@
+#ifndef TOWERBG_H
+#define TOWERBG_H
+
+#include <SDL.h>
+
+struct TowerBG
+{
+	SDL_Surface* buffer;
+	SDL_Surface* buffer_lerp;
+	bool tdrawback;
+	int bypos;
+	int bscroll;
+	int colstate;
+	int scrolldir;
+};
+
+#endif /* TOWERBG_H */