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/eventsets/techrash_u.lua

@@ -9,7 +9,7 @@ return {
             GC.setColor(1,.3,.3,.5-i*.04)
             GC.rectangle('fill',30*L[i]-30,0,30,600)
         end
-        PLY.draw.cancelField(P)
+        PLY.draw.cancelField()
     end,
     hook_drop=function(P)
         local C=P.lastPiece