Currently, the committed signal is emitted after the client has called
wl_surface.commit. However, this breaks with synchronized subsurfaces.

Notably, Firefox splits a web page in a bunch of smaller layers, which
can be backed by wl_subsurface objects.

All the subsurfaces are in the sync mode. If a layer needs to be
repainted, Firefox will commit the corresponding subsurface with a frame
callback.

Since the committed signal is emitted when the wl_surface.commit request
is invoked, kwin will schedule a new frame immediately. Meaning, that it
is quite likely that firefox will have old contents.

The right thing to do would be to schedule a frame when all the ancestors
of the layer subsurface have been committed.

This change re-jitters the commit logic so the committed signal is
emitted when a new state is applied to the surface. It also slightly
cleans up how SubSurfaceInterface::parentCommit() is called.

It will be nice to cleanup the commit logic further by calling the
surface role's commit hook unconditionally, i.e. not check whether it's
a subsurface. But doing so may result in infinite recursions. How to
clean up that is still TBD.