Fix rounding issue

The first prime in each row was being placed a pixel too high, most notably causing the pixel for 2 to appear in the otherwise empty 0 row. (2's y value was being set to `Math.ceil((2 - 2) / mod) = Math.ceil(0) = 0` instead of `Math.floor(2-2)/ mod) + 1 = Math.floor(0) + 1 = 1`)