Fix for "thin lines" bug in all current browsers.

The fix works by applying the translation transform to the whole layer at once,
rounding the zoom level to the nearest 1/256th, and of course rounding any
translation transforms to the nearest pixel.
Use CSS transform rather than SVG transform.
Add initial implementation of D3-based GeoJSON layer.
Add support for rectiles.
Add support for attributes and styles in D3 GeoJSON layer.
Add initial implementation of D3-based TopoJSON layer.