Tests scroll speed rendering background instead of each sprite.  FPS 18.
Drops a FPS counter back in, since it's useful for testing.
Provides variable terrain potential with rows and smoother scrolling.
Tracks icicle positions so they don't overlap.
Removes the background <div> since it's no longer used.
Fixes up pointer events so rocks are selectable.
Removes the FPS counter now that I'm done tweaking rendering.