SL-704 - made avatar hierarchy more consistent by adding lluiavatar for viewer-local avs used in upload previews

SL-704 - attachment surface area now computed in bulk rather than updated incrementally. Same dubious formula.

SL-704 - first pass cleaning up comments. Also added LLControlAvatar::initInstance() as a potential hook for optimizations.

MAINT-7913, MAINT-7817 - fix for animesh attachment corruption after teleporting to a different simulator. Turns out the control avatar (skeleton) was getting caught up in the cleanup for the original region

SL-831 - animesh objects are not required to have any tris. If they have no tris, we don't bother making a control avatar for them.

SL-830, SL-831 - no linking if any of the roots is animated object. isAnimatedObject() just uses check against extended mesh flag of the root. On select, LLVOVolume marks whole linkset for update if animated object.

SL-731, SL-779 - more diagnostics for DebugAnimatedObjects, fixed a problem with recursiveMarkForUpdate