function squaredLength(a) {
  var x = a[0];
  var y = a[1];
  var z = a[2];
  var w = a[3];
  return x * x + y * y + z * z + w * w;
}
Calculates the squared length of a vec4

@param {vec4} a vector to calculate squared length of
@returns {Number} squared length of a