Introduce wlr_scene_output_build_state

When we eventually get rid of the output backing buffer compositors won't be able to just commit whatever during a modeset and expect a buffer to be applied to the state. Therefore, without this fixed the commit will always fail in cases like DRM modeset since no buffer is supplied (learned this the hard way). Scene compositors can use this function to apply a fully rendered and ready to go buffer into an arbitrary output state object.

Fixes #3609 (closed)