Handle cusps

[behdad]

If any of the curves have a cusp, we should chop at the cusp and convert sides separately.

[behdad]

Cusps are where the curve tangent falls to zero. That is, when dx/dt=0 and dy/dt=0. Each of those either has 0, 1, or 2 solutions in the [0,1] range, OR is zero for all values of t. So, the case where both are zero can have 0, 1, 2, or infinitely many solutions. The infinitely-many solution is when the curve is a single point. We can ignore that. For the cases of 1 or 2 cusps, we should just break the curve there. In fact, we should break the curves if any of the master curves has a cusp.

This is extremely unlikely to happen in real fonts though.

[behdad]

For the cases of 1 or 2 cusps, we should just break the curve there.

The case of 2 cusps can only happen if the curve is degenerate, ie. a line.