Experimental feature: skeleton offset

Ozan and DaveDub requested a "pelvis offset" that could be controlled by JS as part of an experiment to make the procedural walk animation even more compelling.  I hacked the feature into a side branch and pointed DaveDub at it.

That was a few weeks ago.  Ozan would now like to play with the feature so I'm creating this PR to bring it into the main codebase for further evaluation.

The feature might not play well with some camera modes since it would move the head position as the avatar walked and would therefore might make the camera view swing about -- use at your own risk.
This is one of the things that makes the feature odd. In order to send the modified position up to the AvatarMixer we have to temporarily copy it into the true position so that the base Avatar::toByteArray() method will correctly pack it.