Improve wayzorder

[zdila]

Railways of lower importance are drawn above those of high importance. See for example https://www.freemap.sk/?map=15/48.714525/21.281784&layers=X where rails with service=siding are drawn above rails without service tag.

Mapping file used can be found at https://github.com/FreemapSlovakia/freemap-mapnik/blob/master/mapping.yaml.

"Main" railways are filtered in Mapnik with [class] = 'railway' and [type] = 'rail' and ([service] = 'main' or [service] = '').