Blender Git Loki

Fix T43671: Playing with Mix Factor of Data Transfer Modifier Is Overwritting Data of Base Mesh

The issue was actually affecting all data types that are not regular CDLayer ones, since by default
DerivedMesh references mesh data (verts, edges, etc. - modifying custom normals often implies
modifying edges' sarpness too).
Modfying edge/face sharpness etc. could directly affect the mesh in this case, **bad**!

So we detect whether we are copying data types that may affect non-CDlayers data, and whether
verts array of org dm is same as mesh one - in which case we copy the DM. This avoids
useless copying in most cases.

Note Edit Normals was quite obviously suffering the same issue.