First attempt at adding a platform.
Platform is wrongly offset to the right.
Circle continues through platform - should bounce.
Circle is located by its centre.
Rectangle is located by its top left corner.
Either of these can be changed by ellipseMode(A) or rectMode(A).
The argument A can be CENTER, RADIUS, CORNER, or CORNERS.
It must be upper case.
Using RADIUS locates the shape by its centre.
The 3rd and 4th arguments are shape_width/2 and shape_height/2