wayland: Restore size when configure size is 0x0

wayland: Restore size when configure size is 0x0

According to xdg_shell, an xdg_surface.configure with size 0x0 should
be interpreted as that it is up to the client to set a size.

When transitioning from maximize or fullscreen state, this means the
client should configure its size back to what it was before being
maximize or fullscreen.

This problem currently only occurs on weston because weston sends a
configure with size 0x0 when transitioning back from maximize or
fullscreen.

bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=762713