For subsurfaces, the new state which includes the input shape is not
applied by the compositor if the subsurface is in effective synchronous
mode.

So we need to apply the input shape once parent surface is in effective
desynchronized mode, which is when it's committed, otherwise the input
shape may never be applied if the widget is not using being_paint() /
end_paint() to draw on its subsurface, like clutter does.

We do that only for empty input shape as those won't need update when
the subsurface is resized, for all other non-empty input shape, the
client still has to use begin_paint()/end_paint() for the input shape to
be applied.

https://bugzilla.gnome.org/show_bug.cgi?id=774534