A statebins-style equal-area tile layout: one square per country, positioned
on a row/col grid derived from country centroids. Used by tile_map().
Format
A tibble with columns iso3c, country, row, col; one row per
country, with row/col unique across the grid.
Details
The grid holds one row for each of the 239 countries in country_meta that
has a bundled centroid; the 10 without one (ALA, BVT, GIB, HKG,
MAC, SJM, TKL, TUV, UMI, VGB – see country_meta) have no tile
and so cannot be drawn by tile_map().