Blender Git Loki

Fix/workaround 'convert object' messing up linked data.

'Convert To...' Object operation has very weird effect of actually
working at obdata level, not object level, which means *all* objects
(even unselected/hidden/in other scenes/...) using same obdata will be
converted to new selected type.

IMHO this is very bad behavior, but... not a bug really, so do not
change this for now.

But at least, do not do that when working on some linked data, else it
leaves Blend file in invalid (incoherent) state until next reload.

So workaround for now is to enforce the 'Keep Original' option when some
linked object/obdata is affected by the operation.

Also fixed somewhat broken usercount handling in Curve->Mesh part.