Progress on systemed#605.

The insight is that very large relations are expensive to clip,
and we can often avoid clipping them at higher zooms.

If a polygon clips to the full extent of a lower zoom tile, then
it will do so for all higher zoom tiles.

This PR adds a `largePolygons` map that tracks this. Any polygon
that covers at least a z9 tile will be tracked, and short-circuited
at higher zooms.

It seems to speed up rendering z6/16/18 (Hudson Bay) dramatically.

Neighboring tiles still have very high CPU, will dig into that next.

...I think? This is the case where the size of the multipolygon is 0.

This happened for Hudson Bay when it overlapped with land.

A possible risk: are there valid cases where boost fails to produce a
polygon at a lower zoom level and so returns 0, but would succeed on
a higher zoom?

Processing the Hudson Bay extract [1]:

master:
real	22m27.753s
user	336m50.481s
sys	0m25.314s

this branch:
real	3m46.838s
user	45m33.959s
sys	0m12.161s

takes 3m46sec.

[1]: https://extract.bbbike.org/?lang=en;sw_lng=-99.269;sw_lat=53.617;ne_lng=-65.244;ne_lat=64.616;format=osm.pbf;city=a%20big%20bay;email=cldellow%40gmail.com;as=76.54103151056417;pg=0.8698404751901266;coords=;oi=1;layers=B000T;submit=;expire=

Oops, I was thinking z9 was the base zoom, so it didn't make
sense to track tiles there--but z9 is the base zoom only for this
bitset, not for the actual map.

New timings:

real	1m51.573s
user	16m31.069s
sys	0m9.744s

This suggests that maybe it's worthwhile to track a secondary hierarchy
from z10..z13, too.

Unfortunately, the bug turned out to be a critical part of the speed. :)

With it fixed, the new runtime is 11m5s. This is still a ~50% decrease,
but not what I was hoping for.

Some additional analysis shows that the frequency of a tile being entirely
excluded or covered is rarer than I initially thought -- it looks like
it's maybe 15-25% for Hudson Bay and James Bay.

I suspect a better savings would be to also keep the unsimplified, uncombined
clip from lower zooms around, and have higher zooms clip against them rather
the raw polygon.

I'll see if I can prototype that.