More precise removal of lasers when they go offscreen in demo game

examples/game/lib/game_objects.dart

@@ -167,7 +167,7 @@ class Laser extends GameObject {
   Laser(GameObjectFactory f, int level, double r) : super(f) {
     // Game object properties
     radius = 10.0;
-    removeLimit = 640.0;
+    removeLimit = _gameSizeHeight + radius;
     canDamageShip = false;
     canBeDamaged = false;
     impact = 1.0 + level * 0.5;