From IRC: CSceneCollisionManager::getPickedNodeBB() unconditionally checks collision against children, so visible children of invisible nodes can be hit.  This seems nonsensical, so we now only check children of visible nodes.

Also clarified the comments on ISceneNode::setVisible() and isVisible(), and added a new method, isTrulyVisible() that recursively checks any Parent nodes as well.

Unit/regression tests updated, verifying that they tested (and failed) with the old behaviour and pass with the new.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@1978 dfc29bdd-3216-0410-991c-e03cc46cb475