Move mapclass r/g/b variables off onto TowerBG

This fixes a bug where if you entered a tower before watching the
credits sequence, the credits sequence would have mismatched text and
background colors.

This bug happened because entering a tower modified the r/g/b attributes
of mapclass, and updated graphics.towerbg, without updating
graphics.titlebg too. Then gamecompleterender() uses the r/g/b
attributes of mapclass.

The solution is to put the r/g/b attributes on TowerBG instead. That
way, entering a tower will only modify the r/g/b attributes used to
render towers, and won't affect the r/g/b attributes used to render the
credits sequence.

Additionally, I also de-duplicated the case-switch that updated the
r/g/b attributes based off of the current colstate, because it got
copy-pasted twice, leading to three instances of one piece of code.

desktop_version/src/TowerBG.h

@@ -12,6 +12,9 @@ struct TowerBG
 	int bscroll;
 	int colstate;
 	int scrolldir;
+	int r;
+	int g;
+	int b;
 };
 
 #endif /* TOWERBG_H */