Blender Git Loki

Fix T46672: Concurrent access to source DM in datatransfer modifier.

There is no real elegant solution here, ideally a modifier shall never *modify*
a source DM, but that would imply much better ways to ensure required data
is available in that source DM, which we do not have currently.

So instead, let's use brute force solution for now and always create a local copy
of our source DM, that we can modify to our heart content!