Generate gears with undercut properly

[jnweiger]

Gears with low tooth count and small pressure angles can cause undercut.

Since commit b0f3707 we produce undercut warnings. The gear we generate does not handle the undercut at all. We should attempt to draw the undercut correctly, if the user chosses to use the gear despite the warning.

(... and we should implement profile shift too, see #9 )

[Neon22]

Do you think we can create the undercuts ?
Its obviously a swept angle like the involute but the math is beyond me.
Doing it by csg style approach looks possible but will be mathematically inexact and take a huge amount of code.
So I'm hoping there is a swept angle approach like the involute itself.