MathUtils.h: Investigate replace/remove Clamp() function in favour of C++17 std::clamp()

As we are now C++17 or greater we should probably investigate replacing calls to Clamp() with std::clamp.

Care may need with both performance and behaviour of non-normals etc to ensure the behaviour remains unchanged - for example ordering of arguments when one of them is a Nan, handling of +0.0 vs -0.0 etc

https://en.cppreference.com/w/cpp/algorithm/clamp.html

At first glance our implementation appears to be directly substitutable.