Sfc_Game change to Sfc_Background

GameSfc class changed to Background. The "use one surface for
everything" idea worked well in my older Pygame apps, but it doesn't
play well with sprites.
Background class better reflects its new scope.

Dirty rect's removed from background, as built-ins in Sprite class can
automatically redraw needed parts of background after a sprite moves.
Also panning of the background has returned, using WASD