window: Initialize saved_rect_fullscreen too

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 (closed) Fixes: a51ad8f9