Renderer v6: headless backend
Same as https://github.com/swaywm/wlroots/pull/2240 but for the headless backend. Supersedes https://github.com/swaywm/wlroots/pull/2268.
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Same as https://github.com/swaywm/wlroots/pull/2240 but for the headless backend. Supersedes https://github.com/swaywm/wlroots/pull/2268.