finally fixed the particle system.

game/state/explosion.hpp

@@ -19,6 +19,8 @@ namespace game {
             , maxAge(maxAge * (1.0 + 0.1*util::randf_0_1()))
             , maxRadius(0.05)
         {
+            static size_t id_counter = 0;
+            id = id_counter++;
         }
 
         const Hit       hit;      // kind of the explosion depends on the hit type
@@ -29,5 +31,7 @@ namespace game {
         // anymore and will disappear afterwards
         const float     maxAge;
         const float     maxRadius; // current radius depends on time.
+
+        size_t          id;
     };
 }