Move leisure=track and attraction=water_slide

[dch0ph]

Fixes #4726

Changes proposed in this pull request:

As discussed in #5041:
Move leisure=track from amenity-line (late in layer stack) to new layer between tunnels and landuse-overlay.
Optimisation of layer ordering from end of road fill to roller coaster.
Move attraction=water_slide from amenity-line to new layer between aeroways and roller coaster.

[Also tweak to highway=ford SQL query to simplify hstore query]

Test rendering with links to the example places:

Link:
Before

After

Ford as linear way

attraction=water_slide

Notes:

The git diff for project.mml is not easy to untangle. Here is the debug output from nik4.py showing the layer ordering (new layers highlighted):
layers=landcover-low-zoom,landcover,landcover-line,water-lines-low-zoom,water-lines,water-areas,ocean-lz,ocean,landcover-area-symbols,marinas-area,water-barriers-line,water-barriers-poly,piers-poly,piers-line,water-barriers-point,bridge,buildings,tunnels,leisure-track,landuse-overlay,barriers,cliffs,ferry-routes,turning-circle-casing,highway-area-casing,roads-casing,highway-area-fill,roads-fill,turning-circle-fill,roads-low-zoom,guideways,bridges,aeroways,waterway-bridges,waterslide,roller-coaster-gap-fill,roller-coaster,aerialways,entrances,golf-line,necountries,admin-low-zoom,admin-mid-zoom,admin-high-zoom,power-minorline,power-line,tourism-boundary,protected-areas,trees,country-names,capital-names,state-names,placenames-medium,placenames-small,stations,junctions,bridge-text,county-names,amenity-points,amenity-line,power-towers,roads-text-ref-low-zoom,roads-text-ref,roads-area-text-name,roads-text-name,paths-text-name,railways-text-name,roads-text-ref-minor,text-poly-low-zoom,text-line,text-point,building-text,interpolation,addresses,water-lines-text,ferry-routes-text,admin-text,protected-areas-text,amenity-low-priority,text-low-priority

It seems to me that the SQL query in amenity-line:

       COALESCE(
       'highway_' || CASE WHEN tags->'ford' IN ('yes', 'stepping_stones') THEN 'ford' END,
       'leisure_' || CASE WHEN leisure = 'slipway' THEN leisure END
        ) AS feature

could be further simplified to:

     CASE
     WHEN tags->'ford' IN ('yes', 'stepping_stones') THEN 'highway_ford'
     WHEN leisure = 'slipway' THEN 'leisure_' || leisure
     END AS feature

?