scenes/level: correctly initialize the level scene

imperium-porcorum.love/scenes/levels/init.lua

@@ -31,10 +31,13 @@ function levelLoad(levelID, missionID)
 end
 
 function LevelScene:new(levelID, missionID)
+  LevelScene.super.new(self)
   level_loadSounds()
   loadSprites()
   previousMenu = previous
   currentLevel:loadMission(levelID, missionID)
+
+  self:register()
 end
 
 function LevelScene:update(dt)