If zpos is present, instead of guessing the zpos from the plane type, just use
it. If two planes have the same zpos, we consider their relative position as
undefined.
We don't handle the case where some planes have zpos set but some others don't.
References: https://github.com/emersion/libhwc/issues/4
Many drivers don't expose the zpos plane property. We can guess it from the
plane type though: primary, overlay, cursor. Except some drivers expose
underlays too: underlays, primary, overlay, cursor. Underlay planes don't have a
special type, they are just marked as overlays. To detect them, check whether
their plane ID is less than the primary plane ID.
References: https://github.com/emersion/libhwc/issues/3
