Provide an API to control item batching/sorting

If one uses QQ2 as a "canvas" for shapes/text (= things with opacity), and these shapes are dynamic (for instance, they change their position at regular intervals), pretty soon they'll realize that QQ2 has awful performances in this scenario because the renderer spend an awful amount of time doing sorting and batching of the geometry.

This is kind of an extreme case, where it'd possibly be cheaper to just not batch at all (trading more draw calls for less time spent batching). If the user is fine with it, the renderer could also disable back-to-front sorting for alpha blending, again alleviating the need for sorting the overlapping translucent geometry.

It'd be nice to have some options on Item to tune sorting/batching on that item's subtree.