Fix getClusters dropping points exactly on the query bbox edge

[spokodev]

Fixes #234.

getClusters(bbox, zoom) silently drops a point whose longitude or latitude falls exactly on an edge of the query bbox, even though the documented bbox [westLng, southLat, eastLng, northLat] is inclusive.

const index = new Supercluster().load(points);
// bbox computed as the exact extent of the points:
index.getClusters([minLng, minLat, maxLng, maxLat], 16);
// the point sitting on the min/max edge is missing from the result

This hits the common pattern of computing the bbox as the extent of the points and then querying with it, which is the scenario in #234.

Cause

Points are stored projected and rounded to Float32 (fround(lngX(lng))), but the getClusters range query is built from full-precision Float64 bounds (lngX(minLng), ...). fround can round a stored coordinate slightly outside its exact double, so a point that should sit on the edge ends up just beyond the inclusive range bound and is excluded.

Fix

Project the query bounds through the same fround the stored coordinates use, so an on-edge point compares equal. fround is already defined in the module. The fix is scoped to getClusters.

Testing

Added a test using the coordinates from #234, where the SW-corner point lies exactly on the bbox edge. On the current code that point is dropped (['a','c','d']); with the fix all four are returned. The rest of the suite stays green (16 tests).

[mourner]

Thanks for the fix! This is superceded by #258 though (no fround anymore).