This fixes a case that was overlooked in
!1036 - when we
have a geometry scale > 1 and Wayland subsurfaces that have an offset
to their parent surface (which is often the case when the toplevel surface
includes decoration/shadows etc.), we have to add extra offset to their
opaque regions so they match their 'visible' location.

This is necessary as `meta_cullable_cull_out_children` moves the coordinate
system during culling, but does not know about geometry scale.

Also, remove the redundant check for `window_actor` - we only hit this code
path if a `window_actor` culls out its children.

!1108