Set background music for title screen
Fix issue between in-game and menu fonts

The in-game font uses a transparent font, but that doesn't work well
with the menus. The menus don't refresh the background each time, so the
transparent text would overlap.

I moved the font module into a class and added a sprite offset that
could be set. Then, in sprites.bmp, I copied the transparent font and
pasted it below, changing the background to the blue color. When the
menus appear, they switch the font offset to point to the start of the
menu (blue-background) font, and then to swith back to the in-game
(transparent) font upon exit.