Don't change PNG palette chunk unnecessarily

[sirjuddington]

From NeuralStunner on the zdoom forums:

I've encountered a singularly annoying behavior where any kind of modification on a single PNG - offset changes or color remap - will muck with the stored palette. Usually just changing which index is used for transparency (it seems to prefer the first unused color that's not #0) but also tends to "upgrade" 16 color PNGs to 256 (where the remainder of the palette is filled in with grayscale values) or even truecolor. What's extra weird about it is that doing these modifications on multiple PNGs at a time leaves their palettes alone, which is the correct/preferred behavior.

https://forum.zdoom.org/viewtopic.php?f=232&t=24955&p=1216331#p1214496