Omit unused roll member
Add virtual destructor

Recently added virtual functions but forgot the virtual destructor. These
virtual functions aren't getting used, but the mechanism may prove valuable.
Add distance parameter to host config

Different stereoscopic configurations have different comfortable viewing
distances. There's no universally correct way to infer this distance from
a host configuration, as the definition of comfort changes from 3D TVs to
HMDs to immersive projection systems. So, we'll do the brute force thing
and make it configurable.

This patch also updates some of the stereo configs with comfortable
distances as needed.