Add built-in dither helpers

[paulstraw]

Is your feature request related to a problem? Please describe.
Ordered dithering should be built in to this lib.

Describe the solution you'd like
The current ditherPatterns.nim that I've set up in the examples is pretty janky. It's not terrible to have something like this around for drawing with PD-native patterns, but there should also be methods for properly using ordered dithering.

Describe alternatives you've considered
N/A

Additional context
See https://potch.me/demos/playdither/ (but probably not error-diffusion methods, at least to start)