picture: allow storing an array of Dav1dITUTT35 entries

Nothing in the spec prevents a Temporal Unit from having more than one Metadata OBU of type ITU-T T.35, so export them as an array instead of only exporting the last one we parse. This is backwards compatible with the previous implementation, as users unaware of this change can ignore the n_itut_t35 field and still access the first (or only) entry in the array as they have been doing until now.