Add a rotate function

engine.js

@@ -19,6 +19,14 @@ class vector {
     this.x += other_vector.x;
     this.y += other_vector.y;
   }
+  rotate(rad) {
+    var cos = Math.cos(rad);
+    var sin = Math.sin(rad);
+    //this.x = Math.round(10000*(this.x*cos-this.y*sin))/10000;
+    //this.y = Math.round(10000*(this.x*sin+this.y*cos))/10000;
+    this.x = this.x*cos-this.y*sin;
+    this.y = this.x*sin+this.y*cos;
+  }
 }
 
 class node {