TrackNode: control render order of added Drawables

[gwaldron]

Discussed in https://github.com/gwaldron/osgearth/discussions/1897

^{Originally posted by keince December 9, 2021} I have a trackNode with an icon and several added drawables within the trackNode to render different types of symbology along with the icon.

Is there a way to make it so the trackNode icon is always on top of its drawables? Right now, the drawables render on top of the icon.

Any help would be appreciated...