Mgv6 mudflow: Avoid partially removed stacked decorations

Recently we started to remove decorations if the dirt below was flowed away,
but this did not check for stacked decorations, causing them to have only
their lowest node removed.
Also, placed mud could partially bury stacked decorations.

Remove 'old_is_water' bool which on testing is never true.
Add new function 'moveMud()' to reduce indentation.
Remove stacked decoration nodes above a removed decoration.
Remove stacked decorations partially buried in placed mud.

src/mapgen_v6.h

@@ -162,6 +162,8 @@ public:
 	int generateGround();
 	void addMud();
 	void flowMud(s16 &mudflow_minpos, s16 &mudflow_maxpos);
+	void moveMud(u32 remove_index, u32 place_index,
+		u32 above_remove_index, v2s16 pos, v3s16 em);
 	void growGrass();
 	void placeTreesAndJungleGrass();
 	virtual void generateCaves(int max_stone_y);