Have Compositor::build_{auto,headless,x11,wayland,drm} actually start the backend

[Timidger]

Currently we are deferring it to the run method. I'm not sure why we are doing that, but it probably isn't necessary to do this especially because there might be race conditions if we acquire those resources late in compositor startup code.

If it shouldn't be done there then we should make another affine type that the Compositor builds out to that can enforce manually trying to start the backend only once. (this will mess with the naming a bit, ultimately the final type should be Compositor so we might want to rename the output of the builder methods).