This feature is useful when replaying debug dumps, as only a very small
subset of tile cells are usually dumped (hence numerous cells are not
known).

The previous name was confusing as another mechanism for debugging also
use the term 'dump' for something not related to interpolator reuse.

Fixes issue #189

Fixes issue #189

The new test for the ancestor method fails, but the failure is probably
related to issue#189 (in fact the issue was identified while testing
this). So we will first solve issue#189 in another branch and merge it
once solved, before continuing working on this branch.

This is an attempt at solving issue #188.