function scaleAndAdd(out, a, b, scale) {
  out[0] = a[0] + b[0] * scale;
  out[1] = a[1] + b[1] * scale;
  out[2] = a[2] + b[2] * scale;
  out[3] = a[3] + b[3] * scale;
  return out;
}
Adds two vec4's after scaling the second operand by a scalar value

@param {vec4} out the receiving vector
@param {vec4} a the first operand
@param {vec4} b the second operand
@param {Number} scale the amount to scale b by before adding
@returns {vec4} out