Implement closed venation

[jasonwebb]

In closed venation, multiple branches can converge on one attractor to create closed loops. Each branch will grow until they enter the attractor's kill radius, and if this kill radius is very small it can look like they are connected even though they technically aren't.

To actually get these converging branches to connected together into a single mesh, we need to create a "patch" that connects each branch tip to the others.