MetaShadowFactory: Add the ability to top-fade a shadow

For attached modal dialogs, we want the shadow to fade out at the top
as if the window was glued to the parent at the top. Add a
shadow-top-fade property to MetaWindowActor and the corresponding
parameter to meta_shadow_factory_get_shadow().

The internal implementation of MetaShadow is adjusted to work
in terms of an "inner border" and "outer border" instead of doing
the calculations in terms of an aggregate border and the spread
of the shadow. The old way of doing things gets clumsy when the
top_fade distance is added in as well.

https://bugzilla.gnome.org/show_bug.cgi?id=592382