Add a script that creates a shield, given colour, offsets, and size
Add a script that creates all the shields required
Improvements to the shields shell script
Replace shields with those generated by the new scripts

There are no visual changes, but it is now easier to make them in the future
Merge branch 'svg_scripts' of https://github.com/pnorman/openstreetmap-carto into pnorman-svg_scripts
Consolidate scripts.
Merge branch 'pnorman-svg_scripts'