Implement NewGRF road stop root sprite group selection

src/newgrf.cpp

@@ -5849,7 +5849,7 @@ static CargoID TranslateCargo(uint8 feature, uint8 ctype)
 		}
 	}
 	/* Special cargo types for purchase list and stations */
-	if (feature == GSF_STATIONS && ctype == 0xFE) return CT_DEFAULT_NA;
+	if ((feature == GSF_STATIONS || feature == GSF_ROADSTOPS) && ctype == 0xFE) return CT_DEFAULT_NA;
 	if (ctype == 0xFF) return CT_PURCHASE;
 
 	if (_cur.grffile->cargo_list.size() == 0) {

src/newgrf_roadstop.cpp

@@ -112,8 +112,31 @@ RoadStopResolverObject::RoadStopResolverObject(const RoadStopSpec *roadstopspec,
 	{
 
 	this->town_scope = nullptr;
-	this->root_spritegroup = (st == nullptr && roadstopspec->grf_prop.spritegroup[CT_DEFAULT] != nullptr)
+
-		? roadstopspec->grf_prop.spritegroup[CT_DEFAULT] : roadstopspec->grf_prop.spritegroup[CT_DEFAULT];
+	CargoID ctype = CT_DEFAULT_NA;
+
+	if (st == nullptr) {
+		/* No station, so we are in a purchase list */
+		ctype = CT_PURCHASE;
+	} else if (Station::IsExpected(st)) {
+		const Station *station = Station::From(st);
+		/* Pick the first cargo that we have waiting */
+		for (const CargoSpec *cs : CargoSpec::Iterate()) {
+			if (roadstopspec->grf_prop.spritegroup[cs->Index()] != nullptr &&
+					station->goods[cs->Index()].cargo.TotalCount() > 0) {
+				ctype = cs->Index();
+				break;
+			}
+		}
+	}
+
+	if (roadstopspec->grf_prop.spritegroup[ctype] == nullptr) {
+		ctype = CT_DEFAULT;
+	}
+
+	/* Remember the cargo type we've picked */
+	this->roadstop_scope.cargo_type = ctype;
+	this->root_spritegroup = roadstopspec->grf_prop.spritegroup[ctype];
 }
 
 RoadStopResolverObject::~RoadStopResolverObject()

src/newgrf_roadstop.h

@@ -110,8 +110,13 @@ struct RoadStopResolverObject : public ResolverObject {
 
 /** Road stop specification. */
 struct RoadStopSpec {
-	// We'll have a default and a fence "cargo". Or maybe just a default one?
+	/**
-	GRFFilePropsBase<NUM_CARGO + 1> grf_prop;
+	 * Properties related the the grf file.
+	 * NUM_CARGO real cargo plus three pseudo cargo sprite groups.
+	 * Used for obtaining the sprite offset of custom sprites, and for
+	 * evaluating callbacks.
+	 */
+	GRFFilePropsBase<NUM_CARGO + 3> grf_prop;
 	RoadStopClassID cls_id;     ///< The class to which this spec belongs.
 	int spec_id;                ///< The ID of this spec inside the class.
 	StringID name;              ///< Name of this stop