Remove `set_position()`, and take position as arg to `draw()`
Send position information to draw() call.
Remove all mutations of background sprites and remove RefCell.

* Position is set immutably through `draw()`
* Map tiles are no longer updated w/ elapsed time.
(Tweak bugs introduced re: drawing hud)
Merge branch 'exp/remove-refcell'
Remove unused coordinates from Sprite's constructor.