Feature request: specify direction of scanning

[plazer1]

It would be nice if we could specify the direction of scanning:

• top-down / bottom-up
• left-to-right / right-to-left
• "serpentine scanning" ‒ horizontal direction of scan alternates between lines

Another, more interesting feature, is a non-constant kernel. For example, we could specify a set of kernels ‒ e.g. A = 7 3 5 1; B = 3 2 3 8; C = 4 4 4 4 ‒ which would be applied to successive pixels in sequence (A B C A B C…) or randomly, etc.

An even more interesting feature, although this is no longer Floyd-Steinberg dithering, would be a multi-pass error distribution, where the error can be pushed to previously processed pixels in the first pass(es), and the last pass would either be a standard forward-only distribution (a la Floyd-Steinberg), or the back-propagated error would simply be discarded.