add gif support

Originally committed as revision 6761 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/img2.c

@@ -51,6 +51,7 @@ static const IdStrMap img_tags[] = {
     { CODEC_ID_FFV1      , "ffv1-img"},
     { CODEC_ID_RAWVIDEO  , "y"},
     { CODEC_ID_BMP       , "bmp"},
+    { CODEC_ID_GIF       , "gif"},
     {0, NULL}
 };