avformat/mxfenc: prefer to use the configured metadata

The metadata company_name, product_name, product_version from input
file will be deleted to avoid overwriting information
Please to test with below commands:
./ffmpeg -i ../fate-suite/mxf/Sony-00001.mxf -c:v copy -c:a copy out.mxf
and
./ffmpeg -i ../fate-suite/mxf/Sony-00001.mxf -c:v copy -c:a copy \
        -metadata company_name="xxx" \
        -metadata product_name="xxx" \
        -metadata product_version="xxx" \
        out.mxf

Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>

fftools/ffmpeg_opt.c

@@ -2641,6 +2641,9 @@ loop_end:
         if(o->recording_time != INT64_MAX)
             av_dict_set(&oc->metadata, "duration", NULL, 0);
         av_dict_set(&oc->metadata, "creation_time", NULL, 0);
+        av_dict_set(&oc->metadata, "company_name", NULL, 0);
+        av_dict_set(&oc->metadata, "product_name", NULL, 0);
+        av_dict_set(&oc->metadata, "product_version", NULL, 0);
     }
     if (!o->metadata_streams_manual)
         for (i = of->ost_index; i < nb_output_streams; i++) {

libavformat/mxfenc.c

@@ -757,8 +757,12 @@ static void mxf_write_identification(AVFormatContext *s)
 {
     MXFContext *mxf = s->priv_data;
     AVIOContext *pb = s->pb;
-    const char *company = "FFmpeg";
-    const char *product = s->oformat != &ff_mxf_opatom_muxer ? "OP1a Muxer" : "OPAtom Muxer";
+    AVDictionaryEntry *com_entry = av_dict_get(s->metadata, "company_name", NULL, 0);
+    AVDictionaryEntry *product_entry = av_dict_get(s->metadata, "product_name", NULL, 0);
+    AVDictionaryEntry *version_entry = av_dict_get(s->metadata, "product_version", NULL, 0);
+    const char *company = com_entry ? com_entry->value : "FFmpeg";
+    const char *product = product_entry ? product_entry->value : s->oformat != &ff_mxf_opatom_muxer ? "OP1a Muxer" : "OPAtom Muxer";
+    const char *product_version = version_entry ? version_entry->value : AV_STRINGIFY(LIBAVFORMAT_VERSION);
     const char *platform = s->flags & AVFMT_FLAG_BITEXACT ? "Lavf" : PLATFROM_IDENT;
     const char *version;
     int length;
@@ -767,7 +771,7 @@ static void mxf_write_identification(AVFormatContext *s)
     PRINT_KEY(s, "identification key", pb->buf_ptr - 16);
 
     version = s->flags & AVFMT_FLAG_BITEXACT ?
-        "0.0.0" : AV_STRINGIFY(LIBAVFORMAT_VERSION);
+        "0.0.0" : product_version;
     length = 100 +mxf_utf16_local_tag_length(company) +
                   mxf_utf16_local_tag_length(product) +
                   mxf_utf16_local_tag_length(platform) +