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 {ReadonlyVec2} a the vector to transform
@param {ReadonlyMat2d} m matrix to transform with
@returns {vec2} out