fix many warnings (#889)

* fix many warnings

some of them can be solved by using a prototype-based system
maybe I can try to use some of these systems to reform backgrounds...

* Update app_mahjong.lua

---------

Co-authored-by: MrZ_26 <1046101471@qq.com>

parts/backgrounds/bg2.lua

@@ -5,7 +5,6 @@ local shader=SHADER.grad2
 local t
 function back.init()
     t=math.random()*2600
-    BG.resize(nil,SCR.h)
 end
 function back.update(dt)
     t=(t+dt)%6200