Investigate palette vs PNG

[bep]

We recently added some code to take the PNG color palette into account when doing a resize, to keep the file sizes in line with the original.

I have checked some before/after:

Original (8 bit, 70kb)

Old resize 640, 180 kb:

New resize 640, 46kb

So the file size is now OK, but the quality in the gradients is not good.

/cc @aitva

[bep]

Adding some dithering seem to fix the issue:

[github-actions]

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