Maptiles for PeriodO maps

Makes tiles from Natural Earth data for use in the PeriodO client. We currently use the cross blended hypso with shaded relief, water, and drainages.

To make tiles:

1. Install GDAL with Python support.
2. Make sure the python3 executable with access to the GDAL libs is in your path.
3. Run make all.

This should download the data from Natural Earth and create a directory with the following structure:

maptiles/
├── 0
│   └──   1  (1×1) tile
├── 1
│   └──  32  (8×4) tiles
├── 2
│   └── 128 (16×8) tiles
└── manifest.json