Not default horizontal collision yet

You can note also, at the 3D lesson 6, that the avatar doesn't have default
horizontal collision (against walls, by example). It's possible the definition
of horizontal collision only by code (Not suported yet for Web-Cobalt. Only for
Croquet-Cobalt. Like we did at "SanMinimum island", for a registered visitor.)

May be in the near future we will have default horizontal collision for the
avatar. It's a decision of the developers' group...