window: keep display size when leaving fullscreen for first time

If the application was started in fullscreen, window geometry has not
been saved, since the window was not realized. We can unfullscreen and
restore 1:1 window to match guest display size with
virt_viewer_display_queue_resize()

https://bugzilla.redhat.com/show_bug.cgi?id=916810

src/virt-viewer-window.c

@@ -519,6 +519,8 @@ virt_viewer_window_leave_fullscreen(VirtViewerWindow *self)
         gtk_window_resize(GTK_WINDOW(priv->window),
                           priv->before_fullscreen.width,
                           priv->before_fullscreen.height);
+    } else {
+        virt_viewer_display_queue_resize(priv->display);
     }
 }