Consider destroying wlr_subsurface with its parent

[emersion]

It seems like the only thing a client can do after a sub-surface's parent has been destroyed is destroying the sub-surface. The protocol says:

If the parent wl_surface is destroyed, the sub-surface is unmapped.

And it doesn't seem like there's any way to set a different parent or map again the surface.

In this case, it would make sense to destroy child sub-surfaces when the parent wl_surface is destroyed.

In any case, what happens should be documented.

Follow-up issue: check what happens with xdg_popup.

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wlroots has migrated to gitlab.freedesktop.org. This issue has been moved to:

https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/1709