Just something that arose from a discussion with Tobias, opening for further discussion.
These are basically elementary shadows ported to Adwaita. That includes: a larger shadow for active window, and a lot smaller shadows for tiled windows.
Other than that, I enabled normal->backdrop decoration transition too, not just backdrop->normal. (though maybe it was disabled because of performance concerns?)
Message dialogs, menus and popovers are unchanged.
It also needs changes in mutter, but probably better to first decide on GTK style before changing that one.