Glacier labels color and thresholds

fixes #1016

* changes the glacier label color to `mix(darken(@glacier, 40%), darken(@glacier-line, 30%), 50%)`.
* changes the airport label color and icon to a violet tone to be better distinguishable from both glacier and waterbody labels.
* changes the size threshold for showing labels to be closer to as it was before.  There are still significantly less labels at z=10 but better than before (where at z=10 almost no mountain glaciers showed up).
* adds an additional rule for showing very large glaciers (>10000 pixel) already at z=8.  Due to the size distribution of actual glaciers this only affects larger icefields and icecaps, only a handful of them outside the polar circles (a few in Iceland, two in Norway, two in Patagonia and a few mostly unnamed ones in Alaska/Canada).  

![glacier labels color](http://www.imagico.de/files/glacier-labels-new.png)
![glacier labels at z=10](http://www.imagico.de/files/glacier-labels-alps.png)