wayland: Emit dummy configure event when resizing while fixed

When a fixed size is active (e.g. the window is maximized), gtk_window_resize() shouldn't take immediate effect, so the request was dropped. This made GTK unhappy if this happened, it will freeze updating the window until it received the new size it demanded.

Handle this by being nice and emitting a dummy GDK_CONFIGURE event with the old size where we previously ignored it. It won't resize the window immediately, so it shouldn't have a visible effect, and the size GTK requested is still saved away for when the window is unmaximized, but emitting the event will make GTK receive the event it expects.

We still drop the request on the floor, e.g. if we still haven't seen the initial configuration, just as we do when actually doing the resize.

Closes: #2907 (closed)