Clustering does not take scaleByDistance into account

Entity clustering of billboards and points does not take into account the `scaleByDistance` property or value, causing items to become clustered when they are visually still quite separated.

Start by locating the entity-clustering implementation and the billboard and point handling for `scaleByDistance`. Reproduce the case described in the issue, then verify that clustering accounts for the visually scaled separation of items.

Written by the indexing model from the issue text.