Convert osgAnimation::RigGeometry to double-buffered custom version

OSG animation formats (e.g. dae) use osgAnimation::RigGeometry and osgAnimation::MorphGeometry, but quoting @AnyOldName3 from !421 (comment 450412434) :

These have dynamic data variance, which tells OSG they're not thread-safe, so to delay the next frame's cull until the current one's draw has dealt with them. That's not ideal as in the worst case, it can halve your framerate. This is discussed here: https://wiki.openmw.org/index.php?title=Rendering_Architecture#Threading_considerations.