function transformMat3(out, a, m) {
  var x = a[0],
      y = a[1],
      z = a[2];
  out[0] = x * m[0] + y * m[3] + z * m[6];
  out[1] = x * m[1] + y * m[4] + z * m[7];
  out[2] = x * m[2] + y * m[5] + z * m[8];
  return out;
}
Transforms the vec3 with a mat3.

@param {vec3} out the receiving vector
@param {vec3} a the vector to transform
@param {mat3} m the 3x3 matrix to transform with
@returns {vec3} out