Make _NET_WM_WINDOW_OPACITY orthogonal to window actor opacity

Using MetaWindowActor.opacity for _NET_WM_WINDOW_OPACITY makes it
difficult to implement effects like fading a window in on map.
Instead, set the opacity on the MetaShadedTexture child and use
it when drawing the shadow.

Since the check MetaWindowGroup does on meta_actor_get_paint_opacity()
no longer covers this, we need to handle the opacity in
meta_window_actor_get_obscured_region() explicitly.

https://bugzilla.gnome.org/show_bug.cgi?id=648613