fix rust/44095 (f64::clamp) todos

[LetsMelon]

A pull request must contain a meaningful improvement to the project.

This PR only fixes TODO's for the f64::clamp rfc.
f64::clampis in rust since 1.50.0 -> doc.rust-lang.org

[RazrFalcon]

This is not really a fix. float::clamp from std will panic if min/max is NaN, while our code will not (in release). So there should be a different approach.

Ideally, we should prove that numbers are finite to begin with, but this would require a significant changes to the resvg.

[LetsMelon]

Oh, I haven't thought about that.
Then maybe replace the TODO's with a comment, why resvg is using a homemade clap function instead of the std.

[RazrFalcon]

Yes, this is an outdated comment/todo, sorry.

[LetsMelon]

No problem, but then I'm going to close this PR.