Add PngDecoder::with_limits

[fintelia]

This isn't the most elegant solution given that other decoders use ImageDecoder::set_limits, but idiosyncrasies of the PNG format make that interface non-ideal. Specifically, finding out whether the image is animated (contains an acTL chunk) or has a "simple alpha channel" (contains a tRNS chunk) potentially requires scanning a nearly unbounded number of chunks. Worse, we currently don't have a Seek bound on the reader, so we have to store all the chunks we read in case we need them later.

Fixes #1788

[fintelia]

@HeroicKatora does this look reasonable to you?

[HeroicKatora]

It kind of does, we'll just see about with_limits and the need to change it at runtime. This is certainly the more cautious approach.