Additional image transforms

[t0mpr1c3]

There's actually a fairly short list of transforms that are worth adding. Apart from the ones we've already implemented, there are (roughly in order of usefulness) scale, crop/expand canvas, posterize, dither, and select colour palette. Not a high priority but we could maybe think about adding some of these when it comes to the web UI.

We could be quite opinionated about the options. Some have useful presets (e.g. scale 1:1 or 4:5 aspect ratio) that add a bit of value. Ordered dithering looks suitable for knitting -- almost like mosaic patterns.

[t0mpr1c3]

See also #593

[t0mpr1c3]

V. Pawlik also mentioned the fact that you currently can't repeat an image to fill a desired width unless that results in a whole number of repeats. Image crop fixes this.