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.
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.
