Ideally the final composition format would have enough precision for all of the planes. I think it'd make sense to use ARGB16161616 if the primary plane uses ARGB8888 and an overlay plane uses ARGB16161616. To simplify the code, maybe it's fine to always use ARGB16161616 for the output, and add getters which fetch an ARGB16161616 row for each supported plane format.
