Feature: setting to indicate snow coverage for arctic climate (replaces snow line height)

Setting the snow coverage (in % of the map) makes a lot more sense
to the human, while still allowing the niche player to set (by
finding the correct %) a snow line height they like. This makes for
easier defaults, as it decoupled terrain height from amount of snow.

Maps can never be 100% snow, as we do not have sprites for coastal
tiles.

Internally, this calculates the best snow line height to approach
this coverage as close as possible.

src/tile_type.h

@@ -29,6 +29,8 @@ static const uint MIN_SNOWLINE_HEIGHT = 2;                     ///< Minimum snow
 static const uint DEF_SNOWLINE_HEIGHT = 10;                    ///< Default snowline height
 static const uint MAX_SNOWLINE_HEIGHT = (MAX_TILE_HEIGHT - 2); ///< Maximum allowed snowline height
 
+static const uint DEF_SNOW_COVERAGE = 40;                      ///< Default snow coverage.
+
 
 /**
  * The different types of tiles.