geom/aabb: add expand observer.

geom/aabb.hpp

@@ -73,6 +73,21 @@ namespace cruft::geom {
         }
 
 
+        ///////////////////////////////////////////////////////////////////////
+        /// Applies a negative offset to lo, and a positive offset to hi, and
+        /// returns the result.
+        ///
+        /// The offset applied is identical for lo and hi, so one may need to
+        /// consider halving the supplied magnitude depending on the use case.
+        aabb<S,T>
+        expand [[nodiscard]] (T mag) const
+        {
+            return {
+                lo - mag,
+                hi + mag
+            };
+        }
+
         ///////////////////////////////////////////////////////////////////////
         /// Adds a constant vector to both the lo and hi points.
         aabb<S,T> operator+ (vector<S,T> v) const