In case we can't do better than the current best allocation even if we manage
to allocate all remaining planes, we bail out early. However the bailout
condition was incorrect and didn't account for the current score.
Sometimes the properties applied on a layer are missing on a plane. Instead of
making the whole allocation fail, just mark the plane as incompatible with the
layer.
Given a primary plane with zpos=0 and overlay planes with zpos 1 2, we want
first to allocate the primary plane, then the furthest overlay plane, then the
remaining one: 0 2 1.
References: https://github.com/emersion/libhwc/issues/3
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
