libweston: create/destroy color profile id generator in a safe point

Currently we create the color profile id generator in
weston_compositor_backends_loaded() and destroy it in
weston_compositor_shutdown().

If something goes wrong and Weston does not start properly, we end up
calling weston_compositor_shutdown() for a struct weston_compositor
whose color profile id generator is NULL, crashing.

This fixes that, creating/destroying the id generator in
weston_compositor_create()/destroy().

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>