When I am running Hedgewars (Worms-like game) on full screen and I try to use Alt+Tab, sometimes (very often) xfwm4 crashes.

* OS: Arch Linux
* xfwm4: from master, currently b1248a70
* Hedgewars: 0.9.25 or 0.9.24.1

Error occurs only when
* Hedgewars game is running full screen (enable Game settings -> Graphics -> Game -> Full screen checkbox),
* xfwm4 shows thumbnails on Alt+Tab (xfwm4 tweaks -> Compositor -> Show windows previews… enabled)

This is the output after crash in console:
```
(xfwm4:1853): Gdk-ERROR **: 07:53:52.045: The program 'xfwm4' received an X Window System error.
This probably reflects a bug in the program.
The error was 'RenderBadPicture (invalid Picture parameter)'.
  (Details: serial 60835 error_code 143 request_code 139 (RENDER) minor_code 8)
  (Note to programmers: normally, X errors are reported asynchronously;
   that is, you will receive the error a while after causing it.
   To debug your program, run it with the GDK_SYNCHRONIZE environment
   variable to change this behavior. You can then get a meaningful
   backtrace from your debugger if you break on the gdk_x_error() function.)

```

I am able to reproduce this bug ONLY if I enforce xfwm4 to show thumbnail of Hedgewars game. Bug does not occur when I disable Alt+Tab thumbnails or when I move to different workspace and then try to move back to previous workspace. 
Also, it seems to be this bug was introduced while porting xfwm4 to gtk3. I am not able to reproduce this with aee8b25 but this bug occurs with 047fbb3 (seems first working commit with gtk3 toolkit).