Feature: setting to indicate desert coverage for tropic climate

This is an indication value; the game tries to get as close as it
can, but due to the complex tropic rules, that is unlikely to be
exact.

In the end, it picks a height-level to base the desert/tropic
line on. This is strictly seen not needed, as we can convert any
tile to either. But it is the simplest way to get started with
this without redoing all related functions.

src/settings_type.h

@@ -298,6 +298,7 @@ struct GameCreationSettings {
 	byte   oil_refinery_limit;               ///< distance oil refineries allowed from map edge
 	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   desert_coverage;                  ///< the amount of desert 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