Blender Git Loki

Depsgraph: remove mesh edit-mode pointer duplication

Share the pointer with the original mesh instead, this matches behavior
of all other objects edit-mode data.

Duplicating the edit-mesh pointer makes updates to edit-mesh require
a COPY_ON_WRITE update, which is currently an expensive operation
(copying the entire mesh).

Notes:

- This change is from 802027f3f8f9a83a77134a2b104a25ff3a4ac013
so the edit-meshes object pointer `BMEditMesh.ob` referenced the COW
version of the object. This pointer has since been removed, so the
copy is no longer needed.

- Having a separate edit-mesh pointer could be used so linked duplicates
could have their own generated meshes. For this to be supported,
many other changes would be needed: see D10920.