Call enterLayer() with two different keys, in order to create
two Graphics that are drawn side by side.
use enterLayer() to modify the (global) p5.js functions to
operate on a Graphics instead of the canvas.
Use a Graphics in order for one object to allow one object to
leave a trail, while the rest of the canvas is fully erased.
One shape leaves a trail (because a semi-transparent color is
drawn over it), while another does not.