Link graph: Store last compression in scaled date ticks

Higher accuracy than using dates at high day legnths

src/saveload/afterload.cpp

@@ -913,6 +913,12 @@ bool AfterLoadGame()
 		_settings_game.linkgraph.recalc_time     *= SECONDS_PER_DAY;
 	}
 
+	/* Convert link graph last compression from date to scaled ticks. */
+	if (SlXvIsFeatureMissing(XSLFI_LINKGRAPH_DAY_SCALE, 4)) {
+		extern void LinkGraphFixupLastCompressionAfterLoad();
+		LinkGraphFixupLastCompressionAfterLoad();
+	}
+
 	/* Load the sprites */
 	GfxLoadSprites();
 	LoadStringWidthTable();

src/saveload/linkgraph_sl.cpp

@@ -125,7 +125,7 @@ public:
 SaveLoadTable GetLinkGraphDesc()
 {
 	static const SaveLoad link_graph_desc[] = {
-		 SLE_VAR(LinkGraph, last_compression, SLE_INT32),
+		 SLE_VAR(LinkGraph, last_compression, SLE_VAR_I64 | SLE_FILE_I32),
 		SLEG_CONDVAR("num_nodes", _num_nodes, SLE_UINT16, SL_MIN_VERSION, SLV_SAVELOAD_LIST_LENGTH),
 		 SLE_VAR(LinkGraph, cargo,            SLE_UINT8),
 		SLEG_STRUCTLIST("nodes", SlLinkgraphNode),