Change: Use per-company group numbers. (#12297)

This is used by the default group name, replacing the use of group index.

src/saveload/afterload.cpp

@@ -3286,6 +3286,17 @@ bool AfterLoadGame()
 		UpdateCompanyLiveries(c);
 	}
 
+	/* Update free group numbers data for each company, required regardless of savegame version. */
+	for (Group *g : Group::Iterate()) {
+		Company *c = Company::Get(g->owner);
+		if (IsSavegameVersionBefore(SLV_GROUP_NUMBERS)) {
+			/* Use the index as group number when converting old savegames. */
+			g->number = c->freegroups.UseID(g->index);
+		} else {
+			c->freegroups.UseID(g->number);
+		}
+	}
+
 	AfterLoadLabelMaps();
 	AfterLoadCompanyStats();
 	AfterLoadStoryBook();