Biomes: Add vertical biome blend (#6853)

Add 'vertical blend' parameter to biome registration that defines how
many nodes above the biome's 'y max' limit the blend will extend.

src/mapgen/mg_biome.h

@@ -67,6 +67,7 @@ public:
 	s16 y_max;
 	float heat_point;
 	float humidity_point;
+	s16 vertical_blend;
 
 	virtual void resolveNodeNames();
 };