Reject cyclic wl_subcompositor.get_subsurface requests

The specification defines that the compositor must raise a BAD_PARENT
protocol error if get_subsurface() is called with a parent which is
a descendant of the surface itself (or if it matches the surface
itself).

Failing to do this could cause an infinite recursion and stack
overflow later.

Pick-to: 6.8 6.11 6.12
Task-number: QTBUG-148611
Change-Id: I6cac060501ea274110e09bec3155f616e6256dc5
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>