libweston, backends: store backend pointer in struct weston_head

Compositor code can use opaque pointer comparison to determine whether
a head belongs to a given backend. Store a backend pointer in struct
weston_head to enable the compositor to select the correct backend
specific output configuration code.

This also allows to use the backend pointer instead of the opaque
backend_id pointer to check whether a head belongs to a backend, so
replace the checks in all to_xyz_head() functions and drop backend_id.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>