Reenable basic glacier rendering in Antarctica

This change re-enables basic rendering of the glacier coverage of Antarctica which got lost after the continent size glacier multipolygon was removed with last years imports.  This implements the first point in the suggestion list on http://wiki.openstreetmap.org/wiki/Polar_Regions_Rendering_Issues, see also #191.

Since apparently it is not possible to limit the extent of shapefile layers in the style (likely either a carto or mapnik limitation) this is done by splitting the shapefiles in preprocessing using ogr2ogr.  Unfortunately this change will therefore require further modifications on the render servers for deployment.

Other than with normal glacier rendering there is no outline drawn since this will only cause problems if additional landcover features are rendered in the future.

This change itself brings only a subtle improvement in the map appearance, it is however prerequisite for any further improvements of antarctic rendering, like in imagico/openstreetmap-carto@9b0bd2adc20e5b586206e48256cd7a4ca1819409 as shown in the following examples:

![glacier rendering example 1](http://www.imagico.de/files/openstreetmap-carto.png)
![glacier rendering example 2](http://www.imagico.de/files/openstreetmap-carto2.png)
![glacier rendering example 3](http://www.imagico.de/files/openstreetmap-carto3.png)