Blender Git Loki

undo experiments: actually write at leat one memchunck per ID.

Checked that before, but missed the fact that the flush was done after a
whole ID type process in the main saving loop, now we actually do it
after every ID when using mem undo file saving.

Besides fixing the stupid issue with lots of unchanged IDs being
detected as changed when reading back the undo step, this should also
help reduce undo memory usage in some cases (although I could not detect
that with some quick test...).