avformat/mxfdec: track metadata sets by their type

We typically are only interesed in a single type of metadata set, so it is
better to keep them separated instead of always filtering for them.

Also use av_dynarray_add for increasing their array.
Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/mxf.h

@@ -55,6 +55,7 @@ enum MXFMetadataSetType {
     GroupOfSoundfieldGroupsLabelSubDescriptor,
     FFV1SubDescriptor,
     JPEG2000SubDescriptor,
+    MetadataSetTypeNB
 };
 
 enum MXFFrameLayout {