dither in x/y, not i/j
dither in x/y, not i/j

The easiest thing to implement was to use the i/j numbers, but they were not
directly related to image x/y coordinates.  This may slow things down a tiny
little bit, but it looks much better.

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython