fate/filter: test gradfun timeline.

tests/filtergraphs/gradfun

 sws_flags=+accurate_rnd+bitexact;
-format=gray, perms=random, gradfun=10:8
+format=gray, perms=random, gradfun=10:8:enable='not(between(n,5,10))'

tests/ref/fate/filter-gradfun-sample

@@ -4,12 +4,12 @@
 0,          2,          2,        1,    76800, 0x45a22d68
 0,          3,          3,        1,    76800, 0x1821013b
 0,          4,          4,        1,    76800, 0xfb17dc4f
-0,          5,          5,        1,    76800, 0x8e7eb458
-0,          6,          6,        1,    76800, 0x0df37d02
-0,          7,          7,        1,    76800, 0x18e4500d
-0,          8,          8,        1,    76800, 0x639316b0
-0,          9,          9,        1,    76800, 0xce00e560
-0,         10,         10,        1,    76800, 0xd533b42c
+0,          5,          5,        1,    76800, 0x07714cf0
+0,          6,          6,        1,    76800, 0x7a5f14df
+0,          7,          7,        1,    76800, 0x421fe837
+0,          8,          8,        1,    76800, 0x39b9af40
+0,          9,          9,        1,    76800, 0x42b47d6b
+0,         10,         10,        1,    76800, 0xb1084cc8
 0,         11,         11,        1,    76800, 0x0772794c
 0,         12,         12,        1,    76800, 0x99544be0
 0,         13,         13,        1,    76800, 0x5f1a16c1