mov: Read alternate absolute path in dref alis

Absolute paths can be stored in alis type 2 and 18 according to my research:
the first is the canonical MacOS filepath, with path level separated by
colons, and the volume name within the filepath, while the second should be the
absolute filesystem path from the mount point.

libavformat/mov.c

@@ -518,12 +518,14 @@ static int mov_read_dref(MOVContext *c, AVIOContext *pb, MOVAtom atom)
                 av_log(c->fc, AV_LOG_DEBUG, "type %d, len %d\n", type, len);
                 if (len&1)
                     len += 1;
-                if (type == 2) { // absolute path
+                if (type == 2 || type == 18) { // absolute path
                     av_free(dref->path);
                     dref->path = av_mallocz(len+1);
                     if (!dref->path)
                         return AVERROR(ENOMEM);
                     avio_read(pb, dref->path, len);
+                    if (type == 18) // no additional processing needed
+                        continue;
                     if (len > volume_len && !strncmp(dref->path, dref->volume, volume_len)) {
                         len -= volume_len;
                         memmove(dref->path, dref->path+volume_len, len);