OSG Stereo broken

In OpenMW versions up to and including 0.47, OSG's stereo rendering has been available (although not perfect). As mentioned here: https://forum.openmw.org/viewtopic.php?f=47&t=7564&p=71425#p71425 it's not working in the master branch right now - the left eye seems to get the 3D scene and the right eye gets the UI. The most likely culprit is probably reverse-Z as that adds an intermediate step that I don't think is stereo-aware.

This might well have implications on @madsbuvi's VR fork, and even if not, he's the only one who's seriously considered stereo rendering in OpenMW, so he's probably got opinions on potential routes forward.

This is fairly low-priority, but the occasional person does use the stereo rendering mode.