Blender Git Loki

Bug fix #21900

Issue with Library linking: if you already have libraries linked in,
and you attempt to link more data, using internally already linked
stuff (like a group with group links) the linkage would fail. It did
resolve on save-load though.

There's a long commented history in this code explaining it. I found
out it was because of manual step-by-step linking versus normal file
reads. In the first case, the "lib pointer map" was missing references.