The scanout plane strictly stacks under all overlay planes, and the
cursor plane above. However, the stacking of overlay planes with respect
to each other is undefined.

We can control the stacking order of overlay planes with the zpos
property, though this significantly complicates plane assignment. In the
meantime, simply disallow assigning a view to an overlay, when it
overlaps another view which is already on an overlay. This ensures
stacking order is irrelevant, since the planes never intersect each
other.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>