Do not map subsurface before parent is mapped

[ammen99]

According to the protocol, subsurfaces become mapped, among other things, when their parent is mapped. Wlroots does such a check if the parent is a subsurface, but not if it is a "toplevel" surface.

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

https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/1735