Support garrisoned enitites defined in map files.

This is adding support to handle garrisoned entities defined in map
files by game engine and atlas allowing future extension for atlas ui
and solving recreation of entites on init in D1958. Also solving
deepfreeze described in D2562.

Differential Revision; https://code.wildfiregames.com/D2597
Ticket: #3008
Patch by: Freagarach
Comments by: elexis
This was SVN commit r23529.

source/graphics/MapWriter.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2019 Wildfire Games.
+/* Copyright (C) 2020 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -39,6 +39,7 @@
 #include "renderer/WaterManager.h"
 #include "simulation2/Simulation2.h"
 #include "simulation2/components/ICmpCinemaManager.h"
+#include "simulation2/components/ICmpGarrisonHolder.h"
 #include "simulation2/components/ICmpObstruction.h"
 #include "simulation2/components/ICmpOwnership.h"
 #include "simulation2/components/ICmpPosition.h"
@@ -341,6 +342,19 @@ void CMapWriter::WriteXML(const VfsPath& filename,
 				if (cmpOwnership)
 					entityTag.Setting("Player", static_cast<int>(cmpOwnership->GetOwner()));
 
+				CmpPtr<ICmpGarrisonHolder> cmpGarrisonHolder(sim, ent);
+				if (cmpGarrisonHolder)
+				{
+					XMLWriter_Element garrisonTag(xmlMapFile, "Garrison");
+					std::vector<entity_id_t> garrison = cmpGarrisonHolder->GetEntities();
+					for (const entity_id_t garr_ent_id : garrison)
+					{
+						XMLWriter_Element garrisonedEntityTag(xmlMapFile, "GarrisonedEntity");
+						garrisonedEntityTag.Attribute("uid", static_cast<int>(garr_ent_id));
+						// ToDo: We can store turret position as well.
+					}
+				}
+
 				CmpPtr<ICmpPosition> cmpPosition(sim, ent);
 				if (cmpPosition)
 				{