evaluate: false redundant matches if overlaps occur on any side already

[bertsky]

The multi-match overlap algorithm (necessary to calculate over- and undersegmentation) still has a glitch: it will create fake/redundant pairings if either side has a segmentation that already overlaps locally. For example, take a page with a GraphicRegion overlapping multiple TextRegions, and evaluate that against itself: the matching will not only produce the 1:1 pairs, but also other matches. That's probably not what we want.