Xfwm4 loses shadows when dragging to restore CSD windows.

Submitted by Hussam Al-Tayeb @hussam

Assigned to Olivier Fourdan @olivier

Link to original bug (#16348)

Description

Steps to reproduce:

1. Maximize a CSD window such as evince.
2. Click on the title bar and drag it to restore (without letting go of the mouse).
3. Notice that the window loses its shadow.

This was reproduced by Brainwash on IRC. It also works as expected on Macro, Mate's Window Manager.

Version: GIT Master