improvement(levels): use a "cross" collision filter for weapons

imperium-porcorum.love/scenes/levels/entities/bullet.lua

@@ -13,7 +13,7 @@ function Bullet:setFilter()
     if     (other.type=="wall")   then
       return 'touch'
     else
-      return nil
+      return 'cross'
     end
   end
 end

imperium-porcorum.love/scenes/levels/entities/weapon.lua

@@ -17,9 +17,9 @@ function Weapon:setFilter()
     elseif (other.type=="block")   then
       return "touch"
     elseif (other.type=="ennemy")   then
-      return "touch"
+      return "cross"
     else
-      return nil
+      return "cross"
     end
   end
 end