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/settings_type.h

@@ -296,7 +296,8 @@ struct GameCreationSettings {
 	uint8  map_y;                            ///< Y size of map
 	byte   land_generator;                   ///< the landscape generator
 	byte   oil_refinery_limit;               ///< distance oil refineries allowed from map edge
-	byte   snow_line_height;                 ///< the configured snow line height
+	byte   snow_line_height;                 ///< the configured snow line height (deduced from "snow_coverage")
+	byte   snow_coverage;                    ///< the amount of snow coverage on the map
 	byte   tgen_smoothness;                  ///< how rough is the terrain from 0-3
 	byte   tree_placer;                      ///< the tree placer algorithm
 	byte   heightmap_rotation;               ///< rotation director for the heightmap