function transformMat4(out, a, m) {
  var x = a[0];
  var y = a[1];
  out[0] = m[0] * x + m[4] * y + m[12];
  out[1] = m[1] * x + m[5] * y + m[13];
  return out;
}
Transforms the vec2 with a mat4
3rd vector component is implicitly '0'
4th vector component is implicitly '1'

@param {vec2} out the receiving vector
@param {vec2} a the vector to transform
@param {mat4} m matrix to transform with
@returns {vec2} out