function transformDirection(m, v, dst) {
    dst = dst || v3.create();

    var v0 = v[0];
    var v1 = v[1];
    var v2 = v[2];

    dst[0] = v0 * m[0 * 4 + 0] + v1 * m[1 * 4 + 0] + v2 * m[2 * 4 + 0];
    dst[1] = v0 * m[0 * 4 + 1] + v1 * m[1 * 4 + 1] + v2 * m[2 * 4 + 1];
    dst[2] = v0 * m[0 * 4 + 2] + v1 * m[1 * 4 + 2] + v2 * m[2 * 4 + 2];

    return dst;
  }
Takes a 4-by-4 matrix and a vector with 3 entries, interprets the vector as a
direction, transforms that direction by the matrix, and returns the result;
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 {Vec3} v The direction.
@param {Vec3} dst optional Vec3 to store result
@return {Vec3} dst or new Vec3 if not provided
@memberOf module:twgl/m4