Stereo shadows are broken again

Here's a screenshot from 0.47 via OSG's built-in stereo rendering that partially didn't work:

The shadows are fine, although it's not actually stereoscopic 3D.

Here's one from whichever 0.48 RC build I installed last:

The shadows are not fine.

For context, here's the same 0.48 RC build in normal mode:

Stereo was temporarily completely broken (#5606 #6261 (closed)) but it was fixed with !1757 (merged), which definitely included some stuff to make shadows work (which had originally been attempted as !273 (closed)).

I'm not sure how to treat this issue. The behaviour has almost certainly regressed within the 0.47-0.48 dev cycle as I don't think @madsbuvi would have declared shadows to be working in stereo if they were this much of a mess. I'm not sure whether we should count it as a regression relative to 0.47, though, as stereo only pretended to work in 0.47, even if shadows worked fine when it was pretending. I'm not entirely sure shadows have been present in a release with properly-working stereo as 0.46 doesn't look 3D any more with settings I thought worked in the past. I can't therefore say whether this needs to be dealt with before 0.48 releases.

As for some investigation, as it looks like a classic LiSPSM maths bug, I turned that off (which is still just a matter of adding an && false to an if), and the shadows lost their anisotropic artefact shape (the lines turned to rectangles) but were still totally awful.