The old VRT_AddV[DF]P() and VRT_RemoveV[DF]P() are "soft deprecated"
and work the same as previous.  (Hard deprecation after next major.)

Replaced by:
    const char *VRT_AddFilter(VRT_CTX, const struct vfp *, const struct vdp *
    void VRT_RemoveFilter(VRT_CTX, const struct vfp *, const struct vdp *);

VRT_CTX is mandatory.

Both kinds of filters can be handled in one go, but the names must be identical.

VRT_AddFilter returns NULL on success, and VRT_fail'ed error message otherwise.

Supersedes #3287