Remove non-relative Seek calls from all functions.

Previously we would seek to the beginning of the io.ReadSeeker. It is much more
flexible to assume that the caller passes the ReadSeeker in an appropriate state.

mp4.go

@@ -62,12 +62,8 @@ type metadataMP4 map[string]interface{}
 // ReadAtoms reads MP4 metadata atoms from the io.ReadSeeker into a Metadata, returning
 // non-nil error if there was a problem.
 func ReadAtoms(r io.ReadSeeker) (Metadata, error) {
-	_, err := r.Seek(0, os.SEEK_SET)
-	if err != nil {
-		return nil, err
-	}
 	m := make(metadataMP4)
-	err = m.readAtoms(r)
+	err := m.readAtoms(r)
 	return m, err
 }