fate: add test for the pad video filter.

tests/lavfi-regression.sh

@@ -37,6 +37,7 @@ do_lavfi "drawbox"            "drawbox=224:24:88:72:#FF8010@0.5"
 do_lavfi "fade"               "fade=in:5:15,fade=out:30:15"
 do_lavfi "null"               "null"
 do_lavfi "overlay"            "split[m],scale=88:72,pad=96:80:4:4[o2];[m]fifo,[o2]overlay=240:16"
+do_lavfi "pad"                "pad=iw*1.5:ih*1.5:iw*0.3:ih*0.2"
 do_lavfi "scale200"           "scale=200:200"
 do_lavfi "scale500"           "scale=500:500"
 do_lavfi "setdar"             "setdar=16/9"

tests/ref/lavfi/pad

+pad                 8cd88a7a224c46f1750e6127445454d5