The goal is to improve clarity and readability, without
introducing big design changes.
Follows the recent obmat to object_to_world refactor: the
similar naming is used, and it is a run-time only rename,
meaning, there is no affect on .blend files.
This patch does not touch the redundant inversions. Those
can be removed in almost (if not all) cases, but it would
be the best to do it as a separate change.