Codechange: Update each town label dimensions once during map gen. (#12742)

This avoids re-layouting the label for each change in population as the town is grown.

src/town_cmd.cpp

@@ -79,6 +79,8 @@ void RebuildTownKdtree()
 	_town_kdtree.Build(townids.begin(), townids.end());
 }
 
+/** Set if a town is being generated. */
+static bool _generating_town = false;
 
 /**
  * Check if a town 'owns' a bridge.
@@ -442,6 +444,8 @@ void ClearAllTownCachedNames()
 static void ChangePopulation(Town *t, int mod)
 {
 	t->cache.population += mod;
+	if (_generating_town) [[unlikely]] return;
+
 	InvalidateWindowData(WC_TOWN_VIEW, t->index); // Cargo requirements may appear/vanish for small populations
 	if (_settings_client.gui.population_in_label) t->UpdateVirtCoord();
 
@@ -1973,6 +1977,8 @@ static void UpdateTownGrowth(Town *t);
  */
 static void DoCreateTown(Town *t, TileIndex tile, uint32_t townnameparts, TownSize size, bool city, TownLayout layout, bool manual)
 {
+	AutoRestoreBackup backup(_generating_town, true);
+
 	t->xy = tile;
 	t->cache.num_houses = 0;
 	t->time_until_rebuild = 10;
@@ -2016,9 +2022,6 @@ static void DoCreateTown(Town *t, TileIndex tile, uint32_t townnameparts, TownSi
 	}
 	t->townnameparts = townnameparts;
 
-	t->UpdateVirtCoord();
-	InvalidateWindowData(WC_TOWN_DIRECTORY, 0, TDIWD_FORCE_REBUILD);
-
 	t->InitializeLayout(layout);
 
 	t->larger_town = city;
@@ -2036,6 +2039,9 @@ static void DoCreateTown(Town *t, TileIndex tile, uint32_t townnameparts, TownSi
 		GrowTown(t);
 	} while (--i);
 
+	t->UpdateVirtCoord();
+	InvalidateWindowData(WC_TOWN_DIRECTORY, 0, TDIWD_FORCE_REBUILD);
+
 	t->cache.num_houses -= x;
 	UpdateTownRadius(t);
 	UpdateTownGrowthRate(t);