We save the window rect before going fullscreen to a dedicated variable,
so we can go back to the correct dimension. We also have a dedicated
variable for returning from other window states, e.g. maximized, and
this one we initialized when creating the MetaWindow. This meant that we
could always rely on this being up to date on X11 windows that were
mapped maximized or fullscreen.

What the commit that introduced the saved rect dedicated for going
unfullscreen missed was to initialize the new saved rectangle too when
creating the MetaWindow. This resulted in windows mapped as fullscreen
often ending up misbehaving when unfullscreening, as mutter would tell
them to unfullscreen to 0x0.

Closes: #1786
Fixes: a51ad8f9
Part-of: <!2210>


(cherry picked from commit 13f35ab0)