Add exception for "gif" in image2 probe() method.

Without this exception files with ".gif" extension by default
recognized as input suitable for image2 demuxer rather than gif.
In order to pass image through gif demuxer it was necessary
to use -f gif option.
This change affected 'make fate' test results because previously
image2 demuxer and gif decoder took only first frame of multiframe
test data, which is no longer true with gif demuxer.
Signed-off-by: Vitaliy E Sugrobov <vsugrob@hotmail.com>

libavformat/img2dec.c

@@ -181,7 +181,7 @@ static int img_read_probe(AVProbeData *p)
             return AVPROBE_SCORE_MAX;
         else if (is_glob(p->filename))
             return AVPROBE_SCORE_MAX;
-        else if(av_match_ext(p->filename, "raw"))
+        else if (av_match_ext(p->filename, "raw") || av_match_ext(p->filename, "gif"))
             return 5;
         else
             return AVPROBE_SCORE_MAX / 2;

tests/ref/lavf/gif

 e6089fd4ef3b9df44090ab3650bdd810 *./tests/data/lavf/lavf.gif
 2906401 ./tests/data/lavf/lavf.gif
-./tests/data/lavf/lavf.gif CRC=0xe5605ff6
+./tests/data/lavf/lavf.gif CRC=0xbf89a246
 022dc66b5068404e88c618ce79d9eb5f *./tests/data/images/gif/02.gif
 ./tests/data/images/gif/%02d.gif CRC=0x032e0034
 81538 ./tests/data/images/gif/02.gif

tests/ref/seek/lavf_gif

-ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:     -1 size:2906401
-ret:-EINVAL    st:-1 flags:0  ts:-1.000000
-ret:-EINVAL    st:-1 flags:1  ts: 1.894167
-ret:-EINVAL    st: 0 flags:0  ts: 0.800000
-ret:-EINVAL    st: 0 flags:1  ts:-0.320000
-ret:-EINVAL    st:-1 flags:0  ts: 2.576668
-ret:-EINVAL    st:-1 flags:1  ts: 1.470835
-ret:-EINVAL    st: 0 flags:0  ts: 0.360000
-ret:-EINVAL    st: 0 flags:1  ts:-0.760000
-ret:-EINVAL    st:-1 flags:0  ts: 2.153336
-ret:-EINVAL    st:-1 flags:1  ts: 1.047503
-ret:-EINVAL    st: 0 flags:0  ts:-0.040000
-ret:-EINVAL    st: 0 flags:1  ts: 2.840000
-ret:-EINVAL    st:-1 flags:0  ts: 1.730004
-ret:-EINVAL    st:-1 flags:1  ts: 0.624171
-ret:-EINVAL    st: 0 flags:0  ts:-0.480000
-ret:-EINVAL    st: 0 flags:1  ts: 2.400000
-ret:-EINVAL    st:-1 flags:0  ts: 1.306672
-ret:-EINVAL    st:-1 flags:1  ts: 0.200839
-ret:-EINVAL    st: 0 flags:0  ts:-0.920000
-ret:-EINVAL    st: 0 flags:1  ts: 2.000000
-ret:-EINVAL    st:-1 flags:0  ts: 0.883340
-ret:-EINVAL    st:-1 flags:1  ts:-0.222493
-ret:-EINVAL    st: 0 flags:0  ts: 2.680000
-ret:-EINVAL    st: 0 flags:1  ts: 1.560000
-ret:-EINVAL    st:-1 flags:0  ts: 0.460008
-ret:-EINVAL    st:-1 flags:1  ts:-0.645825
+ret: 0         st: 0 flags:1 dts: 0.000000 pts: 0.000000 pos:      0 size:117024
+ret:-1         st:-1 flags:0  ts:-1.000000
+ret:-1         st:-1 flags:1  ts: 1.894167
+ret:-1         st: 0 flags:0  ts: 0.790000
+ret:-1         st: 0 flags:1  ts:-0.320000
+ret:-1         st:-1 flags:0  ts: 2.576668
+ret:-1         st:-1 flags:1  ts: 1.470835
+ret:-1         st: 0 flags:0  ts: 0.370000
+ret:-1         st: 0 flags:1  ts:-0.740000
+ret:-1         st:-1 flags:0  ts: 2.153336
+ret:-1         st:-1 flags:1  ts: 1.047503
+ret:-1         st: 0 flags:0  ts:-0.060000
+ret:-1         st: 0 flags:1  ts: 2.840000
+ret:-1         st:-1 flags:0  ts: 1.730004
+ret:-1         st:-1 flags:1  ts: 0.624171
+ret:-1         st: 0 flags:0  ts:-0.480000
+ret:-1         st: 0 flags:1  ts: 2.410000
+ret:-1         st:-1 flags:0  ts: 1.306672
+ret:-1         st:-1 flags:1  ts: 0.200839
+ret:-1         st: 0 flags:0  ts:-0.900000
+ret:-1         st: 0 flags:1  ts: 1.990000
+ret:-1         st:-1 flags:0  ts: 0.883340
+ret:-1         st:-1 flags:1  ts:-0.222493
+ret:-1         st: 0 flags:0  ts: 2.670000
+ret:-1         st: 0 flags:1  ts: 1.570000
+ret:-1         st:-1 flags:0  ts: 0.460008
+ret:-1         st:-1 flags:1  ts:-0.645825