Add clamped math methods to `til` types

Add a clamped sub method to til::point instead of doing it on the outside? I feel like this line should read

const auto offsetPoint = coord.ClampSub(controlOrigin);

Originally posted by @miniksa in https://github.com/microsoft/terminal/pull/5131

I moved this because I thought this comment was a generally good idea. We've got checked math operators defined on the til types already, but there are scenarios where one might want to use clamped math instead. Those callers should be able to use pointA.ClampedAdd(pointB), etc. to be able to do clamped math.

This seems like an easier solution than having some sort of other magic to say "I want a clamped point" that always does clamped math.