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_v7.h

@@ -38,6 +38,7 @@ struct MapgenV7Params : public MapgenParams {
 	u32 spflags = MGV7_MOUNTAINS | MGV7_RIDGES | MGV7_CAVERNS;
 	float cave_width = 0.09f;
 	s16 large_cave_depth = -33;
+	s16 lava_depth = -256;
 	float float_mount_density = 0.6f;
 	float float_mount_height = 128.0f;
 	s16 floatland_level = 1280;