Use a single Vec between Decoder and ZlibStream

[fintelia]

This simplifies buffer management and speeds up decoding. Still some lingering bugs/TODOs. Also need to figure out API considerations as right now this is a breaking change

[Shnatsel]

This Vec can also exceed allocation limits: #286 I was about to work on adding support for limits, but I see there are in-flight PRs touching this code, so I'll wait until this lands.

I'm excited to see this refactoring! Let me know if you want me to do a before/after test run on a large image corpus, or if there's any other way I can help test this.

[anforowicz]

Very interesting - thanks for looking into this! AFAIU fdeflate's requirement for retaining 32kB unmodified decompressed data is met by only calling unfilter on data that was decompressed earlier. Did I get that right? If so, this PR is to some extent reverting the changes from #447 but in exchange this PS enables getting rid of an additional buffer / memcpy hop, right?

[fintelia]

Yes, but this enables another trick: for PNGs with less than 256 KB of uncompressed image data, it does all the decompression in one go. Which means that for those images we don't have to save any space for lookback because we'll be completely done processing the deflate stream by the time we start modifying the data using unfilter.