By default clip is enabled. This means that if the thumbnail would
overlap the "parent" window, it does not get rendered at all. If set
to false it will always be rendered. This is required for window strip
where the complete screen width is used, so overlap does not matter.
This is useful in combination of non-modal TabBox through DBus and
window-strip layout. With DBus the non-modal TabBox can be started
and selecting any item on the strip will end TabBox again.
