Add support for the sRGB chunk.

[de-vri-es]

This PR adds support for the sRGB chunk.

A new type SrgbRenderingIntent is added, and Info now has a srgb: Option<SrgbRenderingIntent> field.

If the decoder encounters an sRGB chunk, it sets the srgb field along with the source_gamma and source_chromaticities field (using predefined values). Any future gAMA or cHRM chunks are ignored (and the data from previous ones is overwritten).

The encoder adds the sRGB chunk along with the pre-defined gAMA and cHRM chunks if the srgb field is set. Manually defined source_gamma and source_chromaticities are ignored in this case.

I believe this conforms to the specification: https://www.w3.org/TR/2003/REC-PNG-20031110/#11sRGB

/edit: CI failure looks like the same Windows/rustup issue, unrelated to the PR contents :(

[de-vri-es]

Note: Maybe SrgbRenderingIntent should be renamed to IccRenderingIntent or just RenderingIntent, as it is more general than its use in the sRGB chunk. It doesn't appear in other places in the PNG specification though.

[HeroicKatora]

CI failure was addressed in #261.

[de-vri-es]

Rebased, CI now passes :)

[HeroicKatora]

Thanks!

[de-vri-es]

Thanks for the review :)