The first feature in a tile can never be dropped, since there is
no previous feature to attach its properties to.
Remove the previous special case that reset the dropping counter
at the first feature within each tile proper (as opposed to the
first feature in each tile, including its buffer, which is now
the one that is guaranteed to be preserved).
This also allows doing the attribute and tilestats all at once
at the end of the tile instead of having to update a feature
to handle the last cluster.
