CavesRandomWalk: Make 'lava_depth' a mapgen parameter

As with 'large_cave_depth', lava depth was previously a fixed y value and
therefore incompatible with the ability to shift terrain vertically.

Add 'lava_depth' mapgen parameter to mgflat, mgfractal, mgv5, mgv7.

src/mapgen.h

@@ -294,6 +294,7 @@ protected:
 	float cavern_limit;
 	float cavern_taper;
 	float cavern_threshold;
+	int lava_depth;
 };
 
 #endif