add placements priority

src/world/generator/StructurePlacement.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <variant>
 #include <stdint.h>
 #include <glm/glm.hpp>
 
@@ -25,3 +26,13 @@ struct LinePlacement {
         : block(block), a(std::move(a)), b(std::move(b)), radius(radius) {
     }
 };
+
+struct Placement {
+    int priority;
+    std::variant<StructurePlacement, LinePlacement> placement;
+
+    Placement(
+        int priority,
+        std::variant<StructurePlacement, LinePlacement> placement
+    ) : priority(priority), placement(std::move(placement)) {}
+};