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

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