Renderer|SectorCluster|Shard: Improving SoC between SectorCluster/Shard

Shard is no longer aware of which map element it was generated for.
SectorCluster provides a mechanism for looking up it's Shards by
their unique identifiers. Revised mechanism for updating Shard's
bias lighting contributions. Cleaned up various APIs.

Todo: The internal management of Shards in SectorCluster should now
be revised to simplify bookkeeping. Perhaps an automatically unique
Shard Id might be useful?