[Tilesathome] The non-rendering of glaciers

[Ævar Arnfjörð Bjarmason]

Currently osmarender does not have a rule to render glaciers, these
are major landmarks in the areas that have them.

Glaciers are traditionally white (#fff) in most maps, osmarenderer
currently renders natural=land as #fff so there's the question of how
they should be colored. White with a thick border of another color?
Blue-ish? Something else?