Use mapnik scale factor

[yhahn]

Currently polyline strokes are not width-doubled. Blue sky:

• Upstream geojson-mapnikify, makizushi use composed svg markers (???)
• Less complexity with @2x raster marker caching, just use a single set of SVG and use the mapnik scale param

For a rainy day.

cc @springmeyer @tmcw

[springmeyer]

SVG would be doable, but for the current iteration I pushed for marker caching in-front of Mapnik because Mapnik does not currently have caching internally for SVG - we literally render the SVG from scratch for each feature processed. The task on the mapnik side would be to selectively cache a rendered SVG for a symbolizer like <MarkerSymbolizer file="some.svg" /> at 1x and 2x scale_factor while avoiding caching a render SVG for a symbolizer like <MarkerSymbolizer file="some.svg" opacity="[some float value]" fill="[another float value]"/ ... etc ... > since the combinations could be infinite.