Improve how we handle vector tiled polygons and lines

[bhousel]

Followup from the recent PMTiles / Vector Tiles work: #1016

• There are still bugs with stitching linestrings and polygons.
• We should probably not do a separate geometry cache per zoom, but rather replace low zoom geometry with high zoom geometry where the tiles are covered

Repeatable example of this is to load Mapbox Streets and check out corner of Collingswood Park

Template URL:
https://api.mapbox.com/v4/mapbox.mapbox-streets-v8/{z}/{x}/{y}.vector.pbf?sku=101HdzJcCmxpq&access_token=TOKEN

Map:

map=18.93/49.23716/-123.03534

(This happens because the park spans several tile boundaries, so it needs to merge several times.)