Fix another geos error in _filter_structures_plane

[weiliangjin2021]

No description provided.

[weiliangjin2021]

Somehow normalize() fixes shapely.errors.GEOSException: TopologyException: side location conflict at ..... This can occur if the input geometry is invalid.. It works without buffer(0), but I just keep it to be safe.

I also shift things around to make it easier to follow.

[tylerflex]

looks good to me, but this kind of shapely manipulation is outside of my knowledge at this point so feel free to get another review if needed.

[momchil-flex]

Approving if tests pass and this fixes the error users encountered, but I don't understand this either. :D @lucas-flexcompute ?

[lucas-flexcompute]

I've always disliked the buffer(0) hack, although that seems to be the only working option in some cases. And I guess normalize does more than the docs imply. There might be other places in the frontend where we use buffer(0) or make_valid that might also benefit from the normalize call. I don't think we need to test that in this PR, but we should remember that the next time we see GEOS/shapely errors.