vf: add three new functions for sending vfctrls down the filter chain

vf_command_all: send command to all filters
vf_command_by_name: send command to named filter
vf_control_by_label: send command to a filter with given label
vf_lavfi: fix ffmpeg deprecation warning for avfilter_graph_parse

use avfilter_graph_parse_ptr for ffmpeg
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: access filter metadata "by-label"
vf-metadata: lua cropdetect example script
vf-metadata: test script