Replace road shields with SVGs

Fixes #693
Fixes #680
Fixes #203
Fixes #202 (mostly)
Fixes #146 (mostly)

Cross-reference #294 mapnik/mapnik#2020

Starts organization for #508

Replace existing PNG road shields with new SVG ones. This allows for easier adjustment of colour and width as well as sharper high-resolution rendering.

At the same time, a number of other shield related bugs are fixed.

To aid in organization, a new shields directory is created.