Please add rtg-math

[cbaggers]

Please add the rtg-math which can be found here https://github.com/cbaggers/rtg-math

This system provides a selection of the math routines most commonly needed for making realtime graphics in lisp.

It differs from sb-cga in 2 main ways

• api design: I'm trying to make a tidier more consistent api with slightly broader scope
• performance: Mine is worse :p .. right now anyway. This will be improved over time

Thanks