Multi-backend support

This allows to load multiple backends simultaneously: one primary backend, which creates the renderer, and optionally one or more secondary backends, which use the renderer created by the primary backend. For backwards compatibility, all backends can be used as the primary backend. Currently the vnc, pipewire, and rdp backends can be loaded as secondary backends. For example:

weston --backends wayland,vnc --vnc-tls-cert tls.crt --vnc-tls-key tls.key