Mapgen: Performance improvement and fixes for updateLiquid (#4065)

- Adds only ~100 nodes per chunk to trans_liquid with similar processing time
- Adds liquid nodes themselves instead of potential solid nodes below them
- CONTENT_IGNORE nodes are interpreted as if they continue their neighborhood
- This allows liquid columns to span multiple chunks without being interrupted
- NOTE: Expects an one-node border in generation chunk without liquid changes

src/mapgen.h

@@ -199,6 +199,10 @@ public:
 	virtual int getSpawnLevelAtPoint(v2s16 p) { return 0; }
 
 private:
+	// isLiquidHorizontallyFlowable() is a helper function for updateLiquid()
+	// that checks whether there are floodable nodes without liquid beneath
+	// the node at index vi.
+	inline bool isLiquidHorizontallyFlowable(u32 vi, v3s16 em);
 	DISABLE_CLASS_COPY(Mapgen);
 };