Add five new types of diffusion dither and expose zig zag to user

This extends the existing dithering code to add Jarvis-Judice-Ninke, Stucki, Atkinson, Burkes and Sierra error diffused dithers. It also exposes the zig zag (serpentine) choice to the user. Diffusion is tweaked to use fixed point for the error values.

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Spotted that serpentine mode wasn't flipping the matrix as it should have - fixed. We now avoid copying error buffers around when just changing index will do. Checking continueTask per-pixel seemed excessive so I've made that less frequent now - per line.

Spotted a problem where dither settings are being lost when OK is hit... [now fixed]