A boolean flag, specifying whether the backdrop is clipped to the
layer.  That's the case when the layer's composite mode is dst-atop
or src-in.

This is a read-only property, derived from the other attributes of
the layer.  We compute its value through a virtual function, so that
GimpGroupLayer will eventually be able to specialize it for pass-
through groups.

The next commit uses this property to actually do something useful.