function transformMat4(out, a, m) {
  var x = a[0],
      y = a[1],
      z = a[2],
      w = a[3];
  out[0] = m[0] * x + m[4] * y + m[8] * z + m[12] * w;
  out[1] = m[1] * x + m[5] * y + m[9] * z + m[13] * w;
  out[2] = m[2] * x + m[6] * y + m[10] * z + m[14] * w;
  out[3] = m[3] * x + m[7] * y + m[11] * z + m[15] * w;
  return out;
}
Transforms the vec4 with a mat4.

@param {vec4} out the receiving vector
@param {vec4} a the vector to transform
@param {mat4} m matrix to transform with
@returns {vec4} out