Make wlr_output_layout_get_box take a box as parameter

[emersion]

trafficstars

Returning a box is strange and inconsistent.

If we choose to return a box, we could at least return a const.

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

https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/812