vizcave and quad-buffered stereo

[AySz88]

Solution - I've noticed that any linkable can be given to <cave>.setTracker(). So my current strategy is to link my tracker with an empty group node, and call <link>.preTrans(). Then I gave the empty node to <cave>.setTracker().

Perhaps this should be documented? The filter extension documentation doesn't make clear that "offset" is a fixed positional offset in the world coordinate frame. Maybe also add something along the lines of "If you need more complex transformations, link an empty group to the tracker(s), apply the appropriate transformations to the link, and then use the group as the new tracker".