Blender Git Loki

Depsgraph fix for editing linked Objects with the other instances being
in other layers (or hidden with local view).

In my search for the absolute minimum of recalculations, changes are only
flushed when they're visible. On changing layers, the tags then are just
set again (for everything that potentially moves) to ensure proper state.

However, it didn't work proper for linked Mesh objects that changed in
editmode, the Derivedmesh callback then accessed memory out of bounds.

The current dependency code was more designed for animation systems...
updating display data should work too, but might need some more tests!

(Thanks Andrea for clear error sample!)