Blender Git Loki

DEBUG_IO: add sanity check on libdata in undo step as well.

There are serious suspicions that weird corruptions faced by studio
artists may happen in undo/redo code, so let's see whether that's the
case.

With this, and when --debug-io arg is passed on startup, the whole lib
data are checked at every undo. This makes undo slower (from two to
three times slower), but it could help us spot better what happens...