ofNode global matrix caching

I went ahead and coded ofNode global matrix caching. Before PR-ing I wanted to discuss whether it's a good approach. You can see the changes here in a feature branch.

What this change does is computing the global matrix only if the local matrix changes. To do so every node keeps not only a parent ofNode* but also a set of children. This way we can flag all children as "dirty" whenever the node's matrix changes.

I've used a vector to make iteration faster, and a map to relate vector indices to the ofNode*. Tried first with an unordered_set<ofNode*> but access was slower.

Drawing, specially when we have a deep hierarchy of nodes is much faster than without caching. Updating (pan, tilt, etc) is a bit slower due to the flagging as dirty process.

I'll do a clean PR when it's polished!

Start with libs/openFrameworks/3d/ofNode.cpp and inspect the linked feature_node_caching branch to understand the proposed global-matrix cache and child tracking. Review the discussion before deciding whether the vector and map approach is acceptable; done means the approach is agreed, polished, and ready for a clean PR.