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/genworld.cpp

@@ -118,6 +118,8 @@ static void _GenerateWorld()
 
 			ConvertGroundTilesIntoWaterTiles();
 			IncreaseGeneratingWorldProgress(GWP_OBJECT);
+
+			_settings_game.game_creation.snow_line_height = DEF_SNOWLINE_HEIGHT;
 		} else {
 			GenerateLandscape(_gw.mode);
 			GenerateClearTile();