(1.21.1) Vertical slab model orientation

[throckmorpheus]

Previously, we were generating all the vertical slab models by rotating the bottom slab model, which made certain vertical double slabs look strange. I've switched it to using the top slab model for the positive half and the bottom slab model for the negative half.

Before:

After:

This rotation method still leaves us with weird-looking differences between how vertical slabs behave as opposed to normal slabs & stairs, so we should probably switch to attempting to load a dedicated vertical slab model and then falling back to the rotated horizontal slab model (so one could fix that with a resource pack), but I don't know how to do that so I'm leaving it here for now.