window: fix initial shadow width for maximized/fullscreen windows

window: fix initial shadow width for maximized/fullscreen windows

To calculate the shadow width, we look at the value of priv->fullscreen
and priv->maximized.
Those fields will have the actual value only after GTK receives back a
window state event though, so they will be wrong in _realize(). Look at
priv->fullscreen_initially and priv->maximize_initially too, to avoid
the size changing right after realize, which would make the window
flicker if maximized at startup.

https://bugzilla.gnome.org/show_bug.cgi?id=747808