roads layer min_zooms at zoom 5 are mix of 3, 4 and 5 instead of just 5.x

[nvkelso]

tl;dr

We only query NE data starting at zoom 5, but the roads min_zoom also includes z3 and z4 values.

Let's do what we did for NE water labels (#1812) and clamp the exported min_zoom to the tile's min_zoom.

We should set:

• 3 to 5.0
• 4 to 5.1
• 5 to 5.2

===

This may be happening in more layers than roads...

• What did you see? Roads receive min_zoom values of 3 and 4 at zoom 5, but are only included in tiles starting at zoom 5.
• What did you expect to see? Either include the roads earlier, or cap the min_zoom of the roads at layer's start_zoom of 5.
• What map location are you having problems with? 5/41.162/-100.195
• Screenshot?

[nvkelso]

Boundaries sees this to an extent on the transition from NE 1:50m data to 1:10m data at zoom 5 when countries like Russia suddenly pop in.

[nvkelso]

This also happens for national_park features in pois layer.

[nvkelso]

/cc @sensescape This bug explains the lack of Natural Earth roads in zoom 3, 4 tiles!