lavf/mov: Write colour matrix "6" for color_space bt470bg.

This matches the demuxer's behaviour.

libavformat/movenc.c

@@ -1608,6 +1608,7 @@ static int mov_write_colr_tag(AVIOContext *pb, MOVTrack *track)
     }
     switch (track->enc->colorspace) {
     case AVCOL_TRC_BT709:     avio_wb16(pb, 1); break;
+    case AVCOL_SPC_BT470BG:
     case AVCOL_PRI_SMPTE170M: avio_wb16(pb, 6); break;
     case AVCOL_PRI_SMPTE240M: avio_wb16(pb, 7); break;
     default:                  avio_wb16(pb, 2);