Displacement map applied during slicing (resolves #8649)

[Poikilos]

This code allows a displacement map to be applied during the slicing stage similar to the vb_fuzzy_skin branch, drastically reducing the overhead of storage, transport, and rework (iterating).

I did this a while back and posted an issue, but didn't really here from people except a few excited people, and one suggested that making a PR would be a better way to bring attention to the feature and get it discussed by the community and considered by maintainers.

Cube mapping is good for many simple tasks like brick walls, which are popularly uploaded to model sites with displacement maps applied. For example, https://www.thingiverse.com/thing:5190970 makes Blender run at 1 frame per 30+ sec on an old i7 while doing any editing. On the contrary, if each brick were a few polygons, the same print detail could be achieved by applying displacement at the slicing stage instead (Keeping a low-poly model for storage, transport, and design iteration).

This feature is often hinted upon or discussed in various videos and forums, but for some reason no slicers (that I know) have implemented it, or even discussed it in official repos or forums. Maybe that is merely since game graphics specialists and engineers do not often share the same skill sets. Down the road maybe even custom UVs would be good, but for now cube mapping is a useful start for various scenarios.

I explain the problem and solution in detail here:

• Displacement mapping a.k.a. bump mapping at the slicing stage (reusing Fuzzy Skin backend code).
  • Cylindrical mapping is also useful, and also not difficult to implement, just translate angle from center of model to 0 through 1 on texture map x axis.