Codechange: Don't save unused NewGRF override mappings. (#9202)

(cherry picked from commit 69e5da0b54)

src/newgrf_commons.h

@@ -218,6 +218,7 @@ public:
 
 	inline uint16 GetMaxMapping() const { return max_new_entities; }
 	inline uint16 GetMaxOffset() const { return max_offset; }
+	inline bool IsValidID(uint16 entity_id) const { return entity_overrides[entity_id] != invalid_ID; }
 };
 
 

src/saveload/newgrf_sl.cpp

@@ -31,6 +31,7 @@ static const SaveLoad _newgrf_mapping_desc[] = {
 void Save_NewGRFMapping(const OverrideManagerBase &mapping)
 {
 	for (uint i = 0; i < mapping.GetMaxMapping(); i++) {
+		if (!mapping.IsValidID(i)) continue;
 		SlSetArrayIndex(i);
 		SlSetLength(4 + 1 + 1);
 		SlObjectSaveFiltered(&mapping.mapping_ID[i], _newgrf_mapping_desc); // _newgrf_mapping_desc has no conditionals