dnd: Get transformed size for scaling before unparenting drag actor

Commit de610a13f1ad1e7e34d4b9a81df58d4da3693059 in mutter made it impossible to access the actors last allocation after unmapping it, this broke the scale-up/down animation when starting a drag.

Fix that animation again by saving the actors transformed size before unparenting (and therefore unmapping) the actor instead of afterwards.