window-wayland: Don't send 1x1 sizes to GTK+ windows

GTK+ requests get_xdg_surface before attaching a buffer, and since it
might take a long time for GTK+ to get around to attaching a buffer and
committing it, our idle for MOVE_RESIZE will kick in beforehand.

And our idle will try to resize the 0x0 window that currently exists,
constrain it to 1x1, which will send a configure event of 1x1 to the
window while it boots up, causing it to awkwardly resize to the minimum
size of the window.

Make sure that in this case, our idle doesn't cause any problems, and
that we fizzle out any idles like this.

The "proper" way to do this would be to delay the creation of the
MetaWindow until a surface is committed, but that's difficult for a
variety of reasons, and might cause unintended issues with focus.