Tweak eval cmp key for more consistent results

[andrews05]

This changes the evaluator to prefer a later image instead of an earlier one in the case of a tie. This gives more consistent results since the baseline is always added last.

I also removed bit depth from the key since the uncompressed size should cover that.

Fixes #649.
(Mostly. I did discover another possible but very rare issue: if multiple colours tie for "most popular edge colour" in the luma sort, the sort may affect which one is actually picked and multiple runs may flip-flop back and forth)

[ace-dent]

@andrews05

... prefer a later image ... since the baseline is always added last.

For my use, preferring the input image (which has been heavily pre-optimised), in case of a tie, is preferred. 👍

... if multiple colours tie... in the luma sort ...

Nice catch! I still ponder whether there is something smarter we can do here around edge colours. I still haven't quite got my head around potential interactions with filtering (beyond f0).... 🤔

[AlexTMjugador]

Thanks, LGTM!