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/tile_type.h

@@ -30,6 +30,7 @@ static const uint DEF_SNOWLINE_HEIGHT = 10;                    ///< Default snow
 static const uint MAX_SNOWLINE_HEIGHT = (MAX_TILE_HEIGHT - 2); ///< Maximum allowed snowline height
 
 static const uint DEF_SNOW_COVERAGE = 40;                      ///< Default snow coverage.
+static const uint DEF_DESERT_COVERAGE = 50;                    ///< Default desert coverage.
 
 
 /**