Fixed|Line: Oversight in Line::considerOneSided()

An id Tech 1 format map with a LINEDEF sporting an unnecessary second
SIDEDEF will typically result in a degenerate BSP leaf on that side.
As such degenerates are not yet "collapsed" during the partitioning
process (and FakeRadio is still using the LineOwner rings) this case
must be specially tested for in Line::considerOneSided().
ConvexSubspace: Cleanup

The geometry generated by the space partitioner is now looking pretty
clean with no partial overlaps and only "duplicate" and/or misordered
line segments (which will be dealt with shortly).

However it is now clear that ultimately resorting to a heuristic based
method of BSP leaf sector selection is inevitable.