Support better color management (RGB profile etc)

Splitting from issue #270

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[may belong to codecs’s repos, but Sqoosh could surface some control] Colour management behaviour is inconsistent. At times broken/dangerous. For PNGs, all input is converted to sRGB , but the sRGB chunk is not set (questionable, although browsers got better at assuming sRGB). For JPGs, RGB profiles are converted, while CMYK ones are a problem, again output is, at times, in sRGB, but there is not profile embedded.

The issue is split from #270; start by reading that issue and checking whether the relevant color-management behavior belongs in Squoosh or a codec repository. Compare the reported PNG and JPEG cases, including RGB/CMYK profiles and embedded sRGB metadata. Done requires an agreed, tested behavior, but this issue does not define the expected output.

Written by the indexing model from the issue text.