Add rounded cube primitive

[Henry00IS]

Such polygons, much smooth! #141

There is an algorithm here that you could take inspiration from.

[ghost]

I suppose that could be used as a bevel feature?

[Falagard]

I wonder if it works well with just one segment.

[Falagard]

https://catlikecoding.com/unity/tutorials/rounded-cube/

[iamarkadyt]

I would like to try my hand on this.
Already started reading related code.
Update:
Pass; sorry.

[Henry00IS]

I created #160 specifically to deal with this PR. You can now chamfer a cube into smithereens.

PolygonFactory:

bool ChamferPolygons(List<Polygon> polygons, List<Edge> edges, float distance, int iterations, out List<Polygon> resultPolygons)

Now that this method is available it should be a lot easier to build this primitive type. The advantage over just chamfering manually is that the size of the edges will remain consistent with the bounds tool... maybe. I found #141 to be quite a challenge.

We have to make some fundamental changes to how SabreCSG tells brushes about invalidations because not every call to Invalidate() is an appropriate moment to update our geometry (like a position change, which causes movement to slow down to a stuttering crawl). It would be great to add another argument like this:

void Invalidate(bool polygonsChanged, InvalidateReason reason)

Then any reasons can inherit from the base abstract class InvalidateReason and possibly even provide additional information through public properties. This is a lot more future-proof than an enum reason as we may run out of flags.