Redefines FORWARD, BACK, FD, and BK to take longer so that you can see the turtle as it moves (otherwise, the turtle moves as fast as it can).
slowness input controls how long it takes for turtle to complete each movement instruction.
The larger the
slowness input, the slower the turtle moves.
SLOWDRAW 0 restores FORWARD, BACK, FD, and BK to their original speed.
slowness input must be a non-negative integer.
SLOWDRAW 100 REPEAT 4 [ FORWARD 100 RIGHT 90 ]