[xmonad] mupdf and llpp return from fullscreen

[Carsten Mattner]

See below for links to the fullscreen code in mupdf.

generic code calling winfullscreen():
http://git.ghostscript.com/?p=mupdf.git;a=blob;f=apps/pdfapp.c;h=d2c28927;hb=HEAD#l789

X11 winfullscreen():
http://git.ghostscript.com/?p=mupdf.git;a=blob;f=apps/x11_main.c;h=941afcfa;hb=HEAD#l325

This might explain why mupdf works with EwmhDesktops's fullscreen
hooks but not Layout.Fullscreen's.