[3.36] window-actor/wayland: Remove subsurface actors on dispose

Backport of !1958 (merged)

I know we normally don't do backports to 3.36 any more. The main reason why I'd like to request a exception here is that this fixes a Mutter crash which Firefox will start to perma-trigger in a hopefully not too far future. As 3.36 is used by the current Ubuntu LTS, which a) gets used by the Firefox CI and b) is likely to be common for a while (even if Wayland is not the default), it would make things easier if this finds its way there. As the surrounding code was also mostly keept in sync, this is also an easy backport.