This ensures that when part of a line is erased,
`UBGraphicsStroke::isNominalLine` will no longer return true. This was
problematic as `UBSvgSubsetAdaptor` uses this to know whether or not to
save a stroke as polyline.
If it saves the stroke as a polyline, then the erased portions of the stroke
reappear after saving.

Therefore, we now force saving of the stroke as a group of polygons when it
has been partially erased.