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 source backwards compatible (But not ABI) with the previous implementation, as users unaware of this change can ignore the new n_itut_t35 field and still access the first (or only) entry in the array as they have been doing until now.

The second patch moves some fields out of Dav1dPicture that have no need to be there, which i only wrote given the first one already needed a major soname bump, so might as well make the best use of it.