window: fix invalid read in computing the input shape

If we are reported only one rectangle in the input shape, we should
not try to read more.

https://bugzilla.gnome.org/show_bug.cgi?id=724257
window: don't set _NET_WM_FULLSCREEN_MONITORS to bogus values

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