code: function(x, y, out) {
          if (!out) {
              out = new Vector2();
          }
          // Invert y;
          y = this._height - y;
  
          var viewport = this.viewport;
          var arr = out.array;
          arr[0] = (x - viewport.x) / viewport.width;
          arr[0] = arr[0] * 2 - 1;
          arr[1] = (y - viewport.y) / viewport.height;
          arr[1] = arr[1] * 2 - 1;
  
          return out;
      }
docs: Convert screen coords to normalized device coordinates(NDC)
  Screen coords can get from mouse event, it is positioned relative to canvas element
  NDC can be used in ray casting with Camera.prototype.castRay methods
  
  @param  {number}       x
  @param  {number}       y
  @param  {clay.Vector2} [out]
  @return {clay.Vector2}