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_v5.h

@@ -35,6 +35,7 @@ struct MapgenV5Params : public MapgenParams
 	u32 spflags = MGV5_CAVERNS;
 	float cave_width = 0.125f;
 	s16 large_cave_depth = -256;
+	s16 lava_depth = -256;
 	s16 cavern_limit = -256;
 	s16 cavern_taper = 256;
 	float cavern_threshold = 0.7f;