add gif support

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@6761 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

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}
 };