Do not render shoulders if zero length
Render edges of a transition with thickness
Draw inside surface of transition offset by transition thickness.
Note the offset is perpendicular to the axis of the rocket, not normal
to the transition surface, so it is not a perfect render, but closer to
correct than before.
Merge pull request #52 from bkuker/kruland-integration-transition-fix

Fixes and improves 3d rendering of transitions