movenc: Addtion of \251cmt field to udta

Allows support of comment in udta for libquicktime, also currently \251des is used
 which is ignored by mov.c.
\251cmt IS currently already picked up by mov.c

References

libquicktime
http://libquicktime.cvs.sourceforge.net/viewvc/libquicktime/libquicktime/src/udta.c?view=markup

Also listed here:
http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/QuickTime.html#UserData

Not listed here, but that`s not entirely surprising, as udta fields are expected to be somewhat arbitary.
http://developer.apple.com/library/mac/#documentation/QuickTime/qtff/QTFFChap2/qtff2.html#//apple_ref/doc/uid/TP40000939-CH204-BBCCFFGDSigned-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/movenc.c

@@ -2078,7 +2078,10 @@ static int mov_write_udta_tag(AVIOContext *pb, MOVMuxContext *mov,
         mov_write_string_metadata(s, pb_buf, "\251alb", "album"      , 0);
         mov_write_string_metadata(s, pb_buf, "\251day", "date"       , 0);
         mov_write_string_metadata(s, pb_buf, "\251swr", "encoder"    , 0);
+        // currently ignored by mov.c
         mov_write_string_metadata(s, pb_buf, "\251des", "comment"    , 0);
+        // add support for libquicktime, this atom is also actually read by mov.c
+        mov_write_string_metadata(s, pb_buf, "\251cmt", "comment"    , 0);
         mov_write_string_metadata(s, pb_buf, "\251gen", "genre"      , 0);
         mov_write_string_metadata(s, pb_buf, "\251cpy", "copyright"  , 0);
     } else {