geom/segment: add addition operator for vectors

geom/segment.hpp

@@ -44,6 +44,18 @@ namespace cruft::geom {
     };
 
 
+    /// Offset a segment by the supplied vector.
+    template <std::size_t S, typename T>
+    segment<S,T>
+    operator+ (segment<S,T> lhs, vector<S,T> rhs)
+    {
+        return {
+            lhs.a + rhs,
+            lhs.b + rhs
+        };
+    }
+
+
     /// Return the squared distance from the closest point of the segment `s`
     /// to the point `p`.
     template <size_t S, typename T>