function transformMat2d(out, a, m) {
  var x = a[0],
      y = a[1];
  out[0] = m[0] * x + m[2] * y + m[4];
  out[1] = m[1] * x + m[3] * y + m[5];
  return out;
}
Transforms the vec2 with a mat2d

@param {vec2} out the receiving vector
@param {vec2} a the vector to transform
@param {mat2d} m matrix to transform with
@returns {vec2} out