matroskadec: don't warn about unknown spherical medata when none is present

track->video.projection.type is set to 0 (a Matroska specific "No spherical
metadata present" value, with no related AVSphericalMapping) by default on
files without the element.

This removes bogus warnings on every single matroska file without Spherical
metadata.
Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/matroskadec.c

@@ -1658,6 +1658,9 @@ static int mkv_parse_video_projection(AVStream *st, const MatroskaTrack *track)
             return AVERROR_INVALIDDATA;
         }
         break;
+    case MATROSKA_VIDEO_PROJECTION_TYPE_RECTANGULAR:
+        /* No Spherical metadata */
+        return 0;
     default:
         av_log(NULL, AV_LOG_WARNING,
                "Unknown spherical metadata type %"PRIu64"\n",