lavfi: make filters less verbose.

libavfilter/asrc_anullsrc.c

libavfilter/avfilter.c

libavfilter/buffersrc.c

libavfilter/fifo.c

libavfilter/vf_aspect.c

libavfilter/vf_blackframe.c

libavfilter/vf_crop.c

libavfilter/vf_cropdetect.c

libavfilter/vf_drawbox.c

libavfilter/vf_fade.c

libavfilter/vf_fieldorder.c

libavfilter/vf_frei0r.c

libavfilter/vf_gradfun.c

libavfilter/vf_hqdn3d.c

libavfilter/vf_libopencv.c

libavfilter/vf_overlay.c

libavfilter/vf_pad.c

libavfilter/vf_scale.c

libavfilter/vf_setpts.c

libavfilter/vf_settb.c