Prior to the DisplayConfig merge, we would set _NET_WM_FULLSCREEN_MONITORS
to (unsigned)-1 when unset. After that, we would have invalid
reads inside meta_screen_monitor_index_to_xinerama_index() (called
with -1).
The way I read the specification, the proper way to indicate
that the window is back to fullscreen on all monitors is to
remove the property, so do that.
Also, add an assertion that meta_screne_monitor_index_to_xinerama_index()
is doing the right thing.
https://bugzilla.gnome.org/show_bug.cgi?id=724258
The goal here is to make MetaWindow represent a toplevel, managed window,
regardless of if it's X11 or Wayland, and build an abstraction layer up.
Right now, most of the X11 code is in core/ and the wayland code in wayland/,
but in the future, I want to move a lot of the X11 code to a new toplevel, x11/.
