Fix for bad bounding shape for multi-root models.

The bounding volume for some models was bad.  The reason was because the models had multiple roots.  For example, this model:

    http://public.highfidelity.io/models/skeletons/leather_jacket.fst

has three roots labled as:

    Hips
    Adam_Head_GeoGroup
    Adam_Body_GeoGroup

The solution is to not try to remember the translation of a single root for computing offsets relative to it, but to remove the translation part of each root when computing the dimensions of the bounding volume --> all child joint transforms will be relative to the origin during the relevant calculations.