maths: add rsqrt convenience function

maths.hpp

@@ -214,6 +214,15 @@ namespace util {
 
     ///////////////////////////////////////////////////////////////////////////
     // exponentials
+
+    // reciprocal sqrt, provided so that we can search for usages and replace
+    // at a later point with a more efficient implementation.
+    inline float
+    rsqrt (float val)
+    {
+        return 1 / std::sqrt (val);
+    }
+
     template <
         typename BaseT,
         typename ExponentT,