wayland.compositor: fix leak of surface data
wayland.compositor: fix leak of surface data

The subsurface logic holds a self-reference to the WlSurface to
track stacking orders of the children relative to the parent. This
self-reference needs be dropped during cleanup to avoid reference
cycles.

This was not the case, and caused every surface user data to be
leaked as it kept a reference to itself (and all of its children).

Smithay/smithay,Smithay/smithay,Smithay/smithay,Smithay/smithay