Special post-processing for rivers

[wipfli]

After merging the water matcher pull request, we found that some rivers got dropped because we increased the minimum polygon area to 10 pixels2.

Here we apply special casing for rivers in the post-processing step, such that river polygons are kept down to 1 pixel2 area.

However, not that sometimes OSM features don't tag stuff as rivers. So in Bogota for example there will still be feature dropping because the merging fails (kind_detail=river is absent). Example: https://www.openstreetmap.org/relation/12757766#map=11/5.6744/-74.4660

[sonarqubecloud]

Quality Gate failed

Failed conditions
9.5% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[wipfli]

Demo: https://maps.protomaps.com/#map=7.46/5.293/-72.859&flavorName=light&lang=en&tiles=https://protomaps.dev/~wipfli/bogota-fix-rivers.pmtiles&show_boxes=true

[bdon]

I think #428 can solve this too

[wipfli]

Closing in favor of #428