Make main branches more continuous

[jasonwebb]

Currently, each time a branching point is reached (that is, when there are two or more children on a node), the "starting" branch/tube is terminated and two new discrete branches/tubes are started. This creates a discontinuity that can be super obvious visually, and problematic for texturing.

I've noticed that in practice, one of the branches always looks like a continuation of the original branch, and the other a smaller offshoot. Rather than starting two entirely new tubes at each branching point, maybe just one tube can be started for the offshoot while the other tube just continues the original tube.