headless: Enable use as a secondary backend

Conditionally create the renderer like other secondary backends.

Now that we have multi-backend support and overlapping outputs, an
interesting use case for the headless backend exists.

We can have a high performance steady state on the drm backend with
all content on scan-out planes, bypassing the renderer. Taking a
screenshot of this would ideally use readback, but some hardware is
incapable of readback, or only capable at certain resolutions.

By using the headless backend as a secondary backend, and creating a
headless output that overlaps the drm outputs, we can take the
screenshot on the headless backend without disrupting the plane
layout on the drm backend.

For this to be efficient, other changes need to be made, but this is
a step in that direction.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>