Fix pixel coordinate to tile coordinate conversion

This addresses issue #9. Although this does not appear to be an issue on Linux
for some reason, it affected a Windows build and the logic appears sound e.g `50
/ 100` correctly resolves to tile `0` whereas `ceil(50.0 / 100)` resolves to
tile `1`.
Merge branch 'master' of github.com:geo-data/cesium-terrain-builder