Feature: [NewGRF] Related Act2 objects for airports and airport tiles.

Airports are similar two stations and industries, both of which have the town as related object.
Airport tiles are similar to industry tiles, which have the industry as related object.
This seems a sensible structure, so let's make it Airport Tile -> Airport -> Town.

src/newgrf_airport.cpp

@@ -14,6 +14,7 @@
 #include "newgrf_text.h"
 #include "station_base.h"
 #include "newgrf_class_func.h"
+#include "town.h"
 
 #include "safeguards.h"
 
@@ -210,6 +211,26 @@ uint32_t AirportResolverObject::GetDebugID() const
 	this->st->airport.psa->StoreValue(pos, value);
 }
 
+/**
+ * Get the town scope associated with a station, if it exists.
+ * On the first call, the town scope is created (if possible).
+ * @return Town scope, if available.
+ */
+TownScopeResolver *AirportResolverObject::GetTown()
+{
+	if (!this->town_scope) {
+		Town *t = nullptr;
+		if (this->airport_scope.st != nullptr) {
+			t = this->airport_scope.st->town;
+		} else if (this->airport_scope.tile != INVALID_TILE) {
+			t = ClosestTownFromTile(this->airport_scope.tile, UINT_MAX);
+		}
+		if (t == nullptr) return nullptr;
+		this->town_scope.reset(new TownScopeResolver(*this, t, this->airport_scope.st == nullptr));
+	}
+	return this->town_scope.get();
+}
+
 /**
  * Constructor of the airport resolver.
  * @param tile %Tile for the callback, only valid for airporttile callbacks.

src/newgrf_airport.h

@@ -15,6 +15,7 @@
 #include "newgrf_class.h"
 #include "newgrf_commons.h"
 #include "newgrf_spritegroup.h"
+#include "newgrf_town.h"
 #include "tilearea_type.h"
 
 /** Copy from station_map.h */
@@ -173,14 +174,23 @@ struct AirportScopeResolver : public ScopeResolver {
 /** Resolver object for airports. */
 struct AirportResolverObject : public ResolverObject {
 	AirportScopeResolver airport_scope;
+	std::unique_ptr<TownScopeResolver> town_scope; ///< The town scope resolver (created on the first call).
 
 	AirportResolverObject(TileIndex tile, Station *st, byte airport_id, byte layout,
 			CallbackID callback = CBID_NO_CALLBACK, uint32_t callback_param1 = 0, uint32_t callback_param2 = 0);
 
+	TownScopeResolver *GetTown();
+
 	ScopeResolver *GetScope(VarSpriteGroupScope scope = VSG_SCOPE_SELF, byte relative = 0) override
 	{
 		switch (scope) {
 			case VSG_SCOPE_SELF: return &this->airport_scope;
+			case VSG_SCOPE_PARENT:
+			{
+				TownScopeResolver *tsr = this->GetTown();
+				if (tsr != nullptr) return tsr;
+				FALLTHROUGH;
+			}
 			default: return ResolverObject::GetScope(scope, relative);
 		}
 	}

src/newgrf_airporttiles.cpp

@@ -214,7 +214,9 @@ static uint32_t GetAirportTileIDAtOffset(TileIndex tile, const Station *st, uint
  */
 AirportTileResolverObject::AirportTileResolverObject(const AirportTileSpec *ats, TileIndex tile, Station *st,
 		CallbackID callback, uint32_t callback_param1, uint32_t callback_param2)
-	: ResolverObject(ats->grf_prop.grffile, callback, callback_param1, callback_param2), tiles_scope(*this, ats, tile, st)
+	: ResolverObject(ats->grf_prop.grffile, callback, callback_param1, callback_param2),
+		tiles_scope(*this, ats, tile, st),
+		airport_scope(*this, tile, st, st != nullptr ? st->airport.type : (byte)AT_DUMMY, st != nullptr ? st->airport.layout : 0)
 {
 	this->root_spritegroup = ats->grf_prop.spritegroup[0];
 }

src/newgrf_airporttiles.h

@@ -44,6 +44,7 @@ struct AirportTileScopeResolver : public ScopeResolver {
 /** Resolver for tiles of an airport. */
 struct AirportTileResolverObject : public ResolverObject {
 	AirportTileScopeResolver tiles_scope; ///< Scope resolver for the tiles.
+	AirportScopeResolver airport_scope;   ///< Scope resolver for the airport owning the tile.
 
 	AirportTileResolverObject(const AirportTileSpec *ats, TileIndex tile, Station *st,
 			CallbackID callback = CBID_NO_CALLBACK, uint32_t callback_param1 = 0, uint32_t callback_param2 = 0);
@@ -52,6 +53,7 @@ struct AirportTileResolverObject : public ResolverObject {
 	{
 		switch (scope) {
 			case VSG_SCOPE_SELF: return &tiles_scope;
+			case VSG_SCOPE_PARENT: return &airport_scope;
 			default: return ResolverObject::GetScope(scope, relative);
 		}
 	}