Major rewrite landcover labelling

This commit changes the rendering of landcover labels.

For the purpose of this commit, with 'landcover label' we mean text connected to
a background colour or pattern rendering, and not connected to an icon.

* All rendered landcover tags now have their name rendered (resolves gravitystorm#537). We
  add labels to the following tags:
  * natural=beach,scrub,grassland,heath,sand,desert (partially resolves gravitystorm#788)
  * highway=services,rest_area (resolves gravitystorm#575)
  * aeroway=apron
  * power=station,generator,substation,sub_station
  * tourism=zoo
  * military=barracks
* The minimum zoom level of labels is now defined based on the number of pixels
  rendered (resolves partially gravitystorm#703, resolves gravitystorm#861, resolves gravitystorm#913).
  Labels are rendered from 3000 pixels (but never earlier than the corresponding
  landuse is rendered).
* Font size now also depends on way_pixels. In other words, larger objects get
  a larger label, in three steps (resolves gravitystorm#308).
* Landuse labels are now rendered oblique, to easier visually tell them apart
  from village and POI labels.
* All labels are rendered in a colour similar to the landuse they belong to
  (using landuse color variables, resolves gravitystorm#56). Also some existing colours
  are changed, in order to make them clearly colourful but still readable.
* The text-halo-radius and text-wrap-width properties are made consistent across
  landuse types.
* Font-size, wrap-width an face-name are now defined by easy to change
  variables.