Vein ore: Fix bug caused by changing perlinmap Y size (#7371)

Because vein ore uses 3D noise (all the other ores use 2D noise) the
perlinmap Y size can be different in different mapchunks when close
to the ore Y limits.
Previously this caused bugs in the vein structure because changes in
perlinmap Y size did not recreate the noise objects.

Delete and recreate the noise objects with the new Y size if Y size
has changed.

src/mapgen/mg_ore.h

@@ -126,6 +126,7 @@ public:
 
 	float random_factor;
 	Noise *noise2 = nullptr;
+	int sizey_prev = 0;
 
 	OreVein() = default;
 	virtual ~OreVein();