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_fractal.h

@@ -35,6 +35,7 @@ struct MapgenFractalParams : public MapgenParams
 	u32 spflags = 0;
 	float cave_width = 0.09f;
 	s16 large_cave_depth = -33;
+	s16 lava_depth = -256;
 	u16 fractal = 1;
 	u16 iterations = 11;
 	v3f scale = v3f(4096.0, 1024.0, 4096.0);