backend/headless: select the rendering device ourselves
backend/headless: select the rendering device ourselves

Backends will eventually stop creating their renderer. To prepare for
this, stop using EGL_PLATFORM_SURFACELESS_MESA in the headless renderer.
Pick a render node using libdrm.

The new allocator/renderer creation logic looks very much like what will
end up in common code.

SirCmpwn/wlroots,swaywm/wlroots,swaywm/wlroots