Draft: render: open platform device render node
Right now we rely on Mesa's renderonly/kmsro to handle split render/display systems. And we have a bunch of special cases in wlroots to accomodate for the fact we don't know about the render node.
However, after a recent discussion (mesa/mesa#9920), it appears we should be able to handle split render/display systems ourselves. If the backend is using a platform display-only device, we can pick a platform render-only device. This should work for all known split render/display SoCs.
Breaking change for custom renderers: wlr_renderer_get_drm_fd()
now requires the FD to be a DRM render node.