Fixes for handling of oversized map dwellings

- Marked large version of H3 Unicorn's Glade as not usable for random
dwelling replacement
- Shifted oversized dwellings - that have at most 2x2 as blocked tile,
but have non-blocked tile column will now be placed correctly
- This fixes incorrect random dwelling replacement of the only oversized
H3 dwelling - Portal of Glory
- Game will now detect & report invalid dwelling templates from mods
- Updated docs to clarify dwellings format

lib/mapObjects/ObjectTemplate.h

@@ -124,6 +124,11 @@ public:
 	// Checks if object can be placed on specific terrain
 	bool canBePlacedAt(TerrainId terrain) const;
 
+	/// Returns number of completely empty rows & columns in template
+	/// Such as shifted wandering monster def's from hota, or Portal of Glory dwelling from H3
+	/// object must be visitable
+	int3 getCornerOffset() const;
+
 	CompoundMapObjectID getCompoundID() const;
 
 	ObjectTemplate();