Rendering the strokes of vector meshes

[Keavon]

Vector meshes generalize paths to have more than two edges connected to a point. That messes with assumptions made by the renderer as part of stroking for caps and joins. We need to devise a strategy for properly rendering such cases.

It would also potentially be worthwhile to control the caps and joins (and miter limit) on a per segment or per anchor basis, even at both ends of just a simple line.

This is currently handled badly, as we can see from the line caps: