copy AVFrame metadata into vf_lavfi priv

store it as an mp_tags object and add VFCTRL_GET_METADATA to access it from elsewhere
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
add autocrop.lua to TOOLS to demonstrate vf-metadata usage