Fix a billion off-by-one errors in merging polygons
Fix a billion off-by-one errors in merging polygons

Man this was a hard one. In my simple test case this fixes all of them. In a more complicated case, I'm finding a few tiny line segments extra. Not perfect, but perhaps acceptable considering how infrequently this gets used.

Contributes to issue CURA-7828.

Ultimaker/CuraEngine,Ultimaker/CuraEngine