function negate(out, a) {
  out[0] = -a[0];
  out[1] = -a[1];
  out[2] = -a[2];
  out[3] = -a[3];
  return out;
}
Negates the components of a vec4

@param {vec4} out the receiving vector
@param {vec4} a vector to negate
@returns {vec4} out