Make polygon simplification aware of shared edges

[e-n-f]

And simplify them the same in both polygons

[mtirwin]

😍

cc/ @ryanbaumann

[e-n-f]

The topology information that comes out this should also enable parcel merging (#169).

[e-n-f]

Working on this in #302

[e-n-f]

There is a missing edge at z1 with

tippecanoe -ps --simplify-polygons-together --no-polygon-splitting -f -z5 -o proper3.mbtiles ne_10m_admin_0_countries.json

[e-n-f]

The missing edge also happens without this new code even running.

grep Russia ne_10m_admin_0_countries.json | time ./tippecanoe -ps --no-polygon-splitting -f -Z1 -z1 -o separate.mbtiles

[e-n-f]

And the missing edge ends up in a different place in the partition branch, even though the output polygon ultimately contains the same points.

One suspicious thing is that Russia here is a polygon ring with 17393 points, which is quite large, and there are strange rendering artifacts as you zoom in. I wonder if GL has an internal limit of 16384 points per polygon ring and this is running into it.

[e-n-f]

Merged. The Russian polygon displays fine in Studio even though mbview has a hard time with it.

[ryanbaumann]

@ericfischer great work! Will test this with some large workflows asap.

[e-n-f]

Hope it works for you @ryanbaumann! Please note you have to specify --detect-shared-borders to turn the feature on.

[e-n-f]

Same problem apparently came up recently in the QGIS world: http://philmikejones.me/2016/09/29/simplify-polygons-without-creating-slithers/