fix: "smearing" of gifs with transparent pixels

[salarkhan]

Fixes: #84

Problem

I tried this with a number of different gifs, seems like it only happened with these stark black/white ones. In the specific case from the linked issue, white pixels from the witch silhouette were being incorrectly marked as transparent when they should remain opaque. The transparency is causing the previous frame's content to show through, creating the smearing effect.

in	out

Fix

So, we have a palette lookup table comprised of bit-crushed RGB values. For extreme colors like pure white (255,255,255), the crushed center calculation: uint32_t R_center = (R & 0xf8) | 4; meant taht for R=255, we'd get 252, aka a slightly different color than what we actually had.

When comparing colors for transparency optimization, the least_dist we got from palette matching might have been larger than it should have been (because we used approximated colors), making it more likely for the transparency choice to "win" the comparison.

By using the actual RGB values instead, we get more accurate color matching against the palette, which means least_dist is more likely to be small for exact color matches. This makes it less likely for the transparency optimization to incorrectly override a good palette color match, especially for extreme colors like pure white or black.

[skidder]

lgtm!