Open line splitting and tagging vs clipping

[bradphelan]

Simply clipping is that act of splitting a set into two and then discarding one or other of the sets. But sometimes it is useful to have both sets. ie: a Split operations rather than a Clip operation. Unfortunately with the current API only Clip in that only one of the two sets during a split is returned.

We have one specific use case for this which is. Often when clipping we want to specify that any gaps of length less than some epsilon should not be clipped and should be filled with the original sub curve that would have been removed. This would be easy to do if clipper returned both sets with the same topology but the split points marked and sub curves tagged.

Currently we have a complex algorithm that tries to match up connected section by running the algorithm twice, once for INTERSECT and the other for DIFFERENCE and trying to align the results which seems like we are trying to recover information that clipper internally has somehow thrown away.