Fix quadratic control point problems in overlap removal routines.

#1710 showed that quadratic splines with mismatched control points were coming out of SCRoundToCluster as called before overlap removal. That call had presumably been added in order to smooth over some of the problems in the overlap removal code, which we have since improved greatly, and it caused significant loss of precision in the resulting shape.

Removing that call revealed problems in SSRemoveTiny (called as part of the overlap removal process). I fixed that.

I subsequently needed to make a few small adjustments to the validation code, but overlap removal seems now to work on quadratic splines without errors.

I did not fix SCRoundToCluster at this time, but it does not get called very often. If this is a higher priority than I have estimated, do tell.

I would appreciate testing by @JonathanMagus and regression testing by @mskala and @tshinnic.