#59 Billboard primitive

[Dominique B.]

The light source are already billboards (not?), so it
might not be to complicated to add a billboard gizmo.

This is not only handy for real-time content creation
but also for things like trees (tree itself as geometry
but large chunks of leavelets are groups together into
images and added to the tress as billboards). I think
SpeedTree uses this technique, too.

I did such node for 3ds max (well indrectly only -
using amanager) and while it has no children, it has
it's own icon. But when assigning children, the icon
disappears (or gets smaller?) Later versions might be
able to contraint to an axis like only rotating along
the z-axis towards the active view etc.