Splats used to require a dedicated row with a special InstanceKey and associated machinery.

With the new store, anything that doesn't match the explicit number of instances given by the row should be treated as a splat:

• within a row with num_instances == 0, a component of unit length is a splat
• within a row with num_instances == 1, a component of unit length is not a splat
• within a row with num_instances == N (N > 1), a component of unit length is a splat

Note: the renderer uses the old InstanceKey::SPLAT constant to drive some related (but different) logic in the selection/hovering subsystems.

Fixing this will fix #1014 in the process.