Move title usages of towerbg to titlebg

With the previous commit in place, we can now simply move some usages of
the previous towerbg to use a separate object instead. That way, we
don't have to mess with a monolithic state, or some better way to phrase
what I just said, and we instead have two separate objects that can
coexist side-by-side.

desktop_version/src/Graphics.cpp

@@ -118,6 +118,7 @@ void Graphics::init()
     footerbuffer = NULL;
     ghostbuffer = NULL;
     towerbg = TowerBG();
+    titlebg = TowerBG();
     trinketr = 0;
     trinketg = 0;
     trinketb = 0;