This moves filtering from the serialization stage to the
tiling stage so that the zoom level can be known to the filter.
The side effect is to carry null attributes much further through
the pipeline than previously.
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).
(The feature count when filtering will be the sum of features
across tiles instead of filters from the original input, since
the filter reader doesn't know what the original input feature
set was.)
