vf-metadata: copy AVFrame metadata into vf_lavfi priv

store it as an mpv_node_map tree and add VFCTRL_GET_METADATA to access it from elsewhere
vf-metadata: add vf-metadata property

This is a read-only property that uses VFCTRL_GET_METADATA
to retrieve the the mp_tags from the vf_lavfi priv structure
vf-metadata: lua cropdetect example script