Don't use std::round in constexpr
Don't use std::round in constexpr

According to the spec, std::round is not constexpr in C++11 yet. Only in C++14 is it constexpr, so we can't use it in a constexpr yet. Instead, use something that is equivalent.

Contributes to issue CURA-4410.

Ultimaker/CuraEngine,Ultimaker/CuraEngine