Don't recursively set bg none on the parent when e.g. moving children

Don't recursively set bg none on the parent when e.g. moving children

To avoid drawing the window background of other windows in the area
where the window was we set the bg to none recursively. However, this
is quite costly it the moved window has many siblings. Furthermore, it
is uncommon that siblings overlap, so this cost has little gain.

So, we only set bg None on the parent, which means that there will
be some more flicker in the uncommon case of overlapping siblings.