function transformNormal(m, v, dst) {
  dst = dst || v3.create();
  var mi = inverse(m);
  var v0 = v[0];
  var v1 = v[1];
  var v2 = v[2];
  dst[0] = v0 * mi[0 * 4 + 0] + v1 * mi[0 * 4 + 1] + v2 * mi[0 * 4 + 2];
  dst[1] = v0 * mi[1 * 4 + 0] + v1 * mi[1 * 4 + 1] + v2 * mi[1 * 4 + 2];
  dst[2] = v0 * mi[2 * 4 + 0] + v1 * mi[2 * 4 + 1] + v2 * mi[2 * 4 + 2];
  return dst;
}
Takes a 4-by-4 matrix m and a vector v with 3 entries, interprets the vector
as a normal to a surface, and computes a vector which is normal upon
transforming that surface by the matrix. The effect of this function is the
same as transforming v (as a direction) by the inverse-transpose of m.  This
function assumes the transformation of 3-dimensional space represented by the
matrix is parallel-preserving, i.e. any combination of rotation, scaling and
translation, but not a perspective distortion.  Returns a vector with 3
entries.
@param {module:twgl/m4.Mat4} m The matrix.
@param {module:twgl/v3.Vec3} v The normal.
@param {module:twgl/v3.Vec3} [dst] The direction. If not passed a new one is created.
@return {module:twgl/v3.Vec3} The transformed normal.
@memberOf module:twgl/m4