create vf-metadata property

This creates a new property "vf-metadata", which accesses metadata exported in AVFrames by lavfi filters. The data is stored in an mp_tags in the vf_lavfi private data structure and accessed via VFCTRL_GET_METADATA. Using the mp_tags type in video/filters/vf_lavfi required moving the related functions and protypes into a more generally accessible location in common/tags.c.h.

An example lua script making use of this metadata is also included in TOOLS.