gcore: Use initializer list for min or max of more than 2 numbers.

[schwehr]

Why?

• Slightly more readable with the form std::min({a, b, c, ...}
• Slightly better performance for non-optimized builds

An example to play with: https://godbolt.org/z/sKfTMz6GE

Spotted reviewing ac95cad / #12814

It's a C++11 feature, so safe to do: https://en.cppreference.com/w/cpp/algorithm/min.html

[schwehr]

Doh! Forgot to run pre-commit

[coveralls]

coverage: 71.118% (-0.003%) from 71.121%
when pulling 821be0f on schwehr:min-max-3plus-args
into 15f3e47 on OSGeo:master.