Only pick when the model has pointer event listeners to hear it

This will keep `view/threejs` from performing an expensive pick when nobody in the model is listening for the data it calculates.

Picks will be performed as before if any node starts listening for a `node3` `pointer*` event. This change doesn't have any provision for stopping the picks once they have started.

@scottnc27603, does this look OK?

@BrettASwift @kadst43 @eric79