Prevent MapBlocks in generation from being unloaded (#16339)

This change prevents issues arising from partial generation of MapChunks, which are liable to be regenerated completely when ungenerated MapBlocks within are encountered.

Co-authored-by: Po Lu <luangruo@yahoo.com>
Co-authored-by: sfan5 <sfan5@live.de>

src/servermap.h

@@ -67,6 +67,7 @@ public:
 	/// @param now current game time
 	void finishBlockMake(BlockMakeData *data,
 		std::map<v3s16, MapBlock*> *changed_blocks, u32 now);
+	void cancelBlockMake(BlockMakeData *data);
 
 	/*
 		Get a block from somewhere.
@@ -169,6 +170,9 @@ protected:
 private:
 	friend class ModApiMapgen; // for m_transforming_liquid
 
+	// extra border area during mapgen (in blocks)
+	constexpr static v3s16 EMERGE_EXTRA_BORDER{1, 1, 1};
+
 	// Emerge manager
 	EmergeManager *m_emerge;