Detect cusps and render them appropriately

[calintat]

Cusps should be rendered with an invisible vertex and vertical-tangent wires. This is similar to half-braids or style 2 in #519.

For algebraic cusps, this can be toggled in the menu (#520). For homotopy cusps which will be possible with invertible generators, we need to detect them. Currently, they are identified as caps and rendered as such (i.e. with horizontal-tangent wires).