lavfi: make AVFilterPad opaque after two major bumps.

avconv.c

libavfilter/af_anull.c

libavfilter/asink_anullsink.c

libavfilter/asrc_anullsrc.c

libavfilter/avfilter.h

libavfilter/buffersrc.c

libavfilter/internal.h

libavfilter/version.h

libavfilter/vf_aspect.c

libavfilter/vf_blackframe.c

libavfilter/vf_boxblur.c

libavfilter/vf_copy.c

libavfilter/vf_crop.c

libavfilter/vf_cropdetect.c

libavfilter/vf_delogo.c

libavfilter/vf_drawbox.c

libavfilter/vf_drawtext.c

libavfilter/vf_fade.c

libavfilter/vf_fieldorder.c

libavfilter/vf_format.c