Voxels are backward

[Gez]

Download this, grab the rocket launcher, fire a rocket and freeze the game to watch it.
As these screenshots show, it's correct in ZDoom, but the rocket is backward in GZDoom.

[Gez]

I fixed this in true Star Trek version by inverting the polarity of the tachyon emitters FVoxelModel::AddFace. Basically, instead of -x and +y, now it's +x and -y. It seems to work better this way: the rocket is no longer backward, the hanging corpses now face towards the center of the room instead of toward the wall in the test room, and the bits that stick out of the switches now properly stick out toward you rather than being against the wall.

Since it seems to work correctly now, I've committed it (r1175). If that wasn't the proper fix and something different was needed, feel free to revert and proceed. But for now, I'll consider this fixed.