avformat/utils: add gif to tb_unreliable()

This improves the used timebases when gif is the source
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -2868,6 +2868,7 @@ static int tb_unreliable(AVCodecContext *c)
         // c->codec_tag == AV_RL32("XVID") ||
         c->codec_tag == AV_RL32("mp4v") ||
         c->codec_id == AV_CODEC_ID_MPEG2VIDEO ||
+        c->codec_id == AV_CODEC_ID_GIF ||
         c->codec_id == AV_CODEC_ID_H264)
         return 1;
     return 0;

tests/ref/fate/gif-disposal-background

-#tb 0: 1/100
+#tb 0: 1/2
 0,          0,          0,        1,    10368, 0x8b200cc8
-0,         50,         50,        1,    10368, 0x4e208e4f
-0,        100,        100,        1,    10368, 0x044dba8b
-0,        150,        150,        1,    10368, 0x1d617e09
-0,        200,        200,        1,    10368, 0xae38a6d2
+0,          1,          1,        1,    10368, 0x4e208e4f
+0,          2,          2,        1,    10368, 0x044dba8b
+0,          3,          3,        1,    10368, 0x1d617e09
+0,          4,          4,        1,    10368, 0xae38a6d2

tests/ref/fate/gif-disposal-restore

-#tb 0: 1/100
+#tb 0: 1/1
 0,          0,          0,        1,   112320, 0xb8afe429
-0,         10,         10,        1,   112320, 0xae588a4b
-0,        310,        310,        1,   112320, 0xccdd27b7
+0,          3,          3,        1,   112320, 0xccdd27b7

tests/ref/lavf/gif

 e35f5ea283bbcb249818e0078ec72664 *./tests/data/lavf/lavf.gif
 2011766 ./tests/data/lavf/lavf.gif
-./tests/data/lavf/lavf.gif CRC=0x0dc5477c
+./tests/data/lavf/lavf.gif CRC=0x2429faff