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.
Another thing that makes the feature questionable is that it probably doesn't play well with some camera modes.  I've tried to preemptively solve this problem by disabling enabling it when in first person -- disabled for both local and remote clients.  That is, it works for everybody when you're watching your own avatar walk around, but for nobody when you're walking around in first person.