vf-metadata: copy AVFrame metadata into vf_lavfi priv

store it as mp_tas 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 mp_tags metadata from a filter specified by label
vf-metadata: lua cropdetect example script
vf-metadata: test script
af_lavfi: fix graph parse deprecation warning
af_volume: fix clang -Wsometimes-uninitialized