Remove redundant events from `wupget:UnsyncedHeightMapUpdate`

If a small rectangle completely fits within a big one it's sometimes sent redundantly anyway. Perhaps batch it or something.

What should happen to near-overlap (e.g. whether or not red+green should be merged to blue in the example below) is left unspecified.

Start by tracing the `wupget:UnsyncedHeightMapUpdate` event and how rectangle updates are queued or batched. Reproduce a case where a small rectangle is fully contained by a larger one, then define completion as eliminating the redundant event while preserving the currently unspecified near-overlap behavior until clarified.

Written by the indexing model from the issue text.