Add features helping transform/visibility propagation

Hi! No expectations here since I'm opening a completely unsolicited PR.

Anyway, I have been using the library in a rendering engine experiment, I like the quite simple to-the-point design and would primarily like to add a couple of things to more easily add transform & visibility propagation when working with dirty flags for these, i.e. one wants to mark sub-trees as dirty so that attributes are propagated selectively, while unchanged parts of the graph are skipped.

Also added a mutable version of iter_out_of_order.

BR Jerker

Hey! I'm on vacation and will take a look when I am back. Thank you for the work!

Hey! I'm on vacation and will take a look when I am back. Thank you for the work!

Great, no rush! I have zero ego here and just want/need these kind of features, I do not care much about the attribution or so if you'd feel like implementing things in a different way. But am also available to modify based on feedback.

I also drafted some ability to iterate from node with access to node index - the below is just a PR to my fork ATM to not clutter this one even more, since this current PR is from my master branch. https://github.com/tville/scene-graph/pull/4/files