gdk: Always get shadow width via GdkToplevelSize

gdk: Always get shadow width via GdkToplevelSize

This removes the gdk_surface_set_shadow_width() function and related
vfuncs. The point here is that the shadow width and surface size can now
be communicated to GDK atomically, meaning it's possible to avoid
intermediate stages where the surface size includes the shadow, but
without the shadow width set, or the other way around.