mp4: Merge parse_leaf_atom() into parse_sub_atoms().

mp4: Merge parse_leaf_atom() into parse_sub_atoms().

This gets rid of the distinction between atoms with and without
subatoms, which was confusing because some atoms "without" subatoms
in fact do contain subatoms, we just did not want to parse them
recursively in parse_sub_atoms().

With this weirdness gone, we may move on to simplify the atoms enum and
atom_name_to_type() further, but this is left to a subsequent patch.