[Tile-serving] [mod_tile] Map parameterization should be reverted (#104)

[Dane Springmeyer]

/cc @jburgess777 

Looking into ttps://github.com/mapnik/mapnik/issues/2965, I wondered if renderd was doing anything unsafe.

I noticed that https://github.com/openstreetmap/mod_tile/commit/c4aabc570103e72876e9889a20396e719f52bca4 added mutation of map objects per render. While this might work I think it should be reverted for at least these reasons:

 - A copy of the `mapnik.Map` is made every time a tile is rendered even if no `parameterize_function` is set. This is potentially very expensive.
 - I think parameterization would be much better done in XML before loading the map. Ideally maps should not be modified after being loaded.
 - If per render changes to styling truly need to be made, then a much better design would be to leverage Mapnik 3.x variables: https://github.com/mapnik/mapnik/issues/2553

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Reply to this email directly or view it on GitHub:
https://github.com/openstreetmap/mod_tile/issues/104
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