function min(out, a, b) {
  out[0] = Math.min(a[0], b[0]);
  out[1] = Math.min(a[1], b[1]);
  out[2] = Math.min(a[2], b[2]);
  out[3] = Math.min(a[3], b[3]);
  return out;
}
Returns the minimum of two vec4's

@param {vec4} out the receiving vector
@param {vec4} a the first operand
@param {vec4} b the second operand
@returns {vec4} out