added optional output argument to Vector.rotate

Author: liabru

src/geometry/Vector.js

@@ -62,14 +62,16 @@ module.exports = Vector;
      * @method rotate
      * @param {vector} vector
      * @param {number} angle
-     * @return {vector} A new vector rotated about (0, 0)
+     * @param {vector} [output]
+     * @return {vector} The vector rotated about (0, 0)
      */
-    Vector.rotate = function(vector, angle) {
+    Vector.rotate = function(vector, angle, output) {
         var cos = Math.cos(angle), sin = Math.sin(angle);
-        return {
-            x: vector.x * cos - vector.y * sin,
-            y: vector.x * sin + vector.y * cos
-        };
+        if (!output) output = {};
+        var x = vector.x * cos - vector.y * sin;
+        output.y = vector.x * sin + vector.y * cos;
+        output.x = x;
+        return output;
     };
 
     /**