Blender Git Loki

Preparing merge of id_override_static: add BKE_id_swap().

This function swaps the memory content of two data-blocks (of same type
obviously), while preserving most of the ID 'header' itself.

It is intended to be used to quickly and easily replace the data of an
existing ID by another one, presumably a temporary 'working' one,
without having to suffer from things like name changes,
registering/removing from Main database, etc.