CI: Rewrite set_image for Spectra 6 to handle more edge cases

[Gadgetoid]

We've had various problems with the image remapping code (particularly where P mode images have more than six colours), most of which was very, very old code I wrote back when I had much less idea what I was doing.

This rewrite should fix all cases by avoiding as much special casing as possible. The one exception is where a P mode image has no palette at all, and thus no colour information. For example the Hello World example uses palette indexes but never creates a palette (image.palette.colors == {}) so attempting to quantize it has very bizarre results.

1. Test for IndexError - index 254 out of bounds for axis 0 with size 6 #227
2. Test fix color-remapping for images that are already in platte mode #221 fixes this case (it does not)
3. Test fix color-remapping for images that are already in platte mode #221 looks good! (N/A)
4. New goal: Rewrite set_image

[bablokb]

I have tested it and now the pre-dithered image is left alone.