Work in progress on tiling gridded data

[e-n-f]

Experiments for mapbox/vector-tile-spec#50

To test:

git checkout grids
make clean
make -j
./tippecanoe -z4 -f -o grid.mbtiles tests/grid/in.json

There is no rendering support yet for grids, but you can do

./tippecanoe-decode grid.mbtiles | less

to see what data from the sample grid ends up in each tile.

Still to come: reading and writing PNG and JPEG format instead of just arrays of numbers.