Enabling side-scrolling

Today's diff is somewhat small, but it changes a logical aspect of the game that's pretty important for this kind of game : today I added side-scrolling...

so you can now move the character right/left and actually watch the background scroll towards the sides as the character moves.

this required adding a high-level concept of a 'map' that extends beyond the viewport. when drawing, i transform the camera to draw the entire map, then undo the transform to draw the character.