规范代码，大量xxx=nil换成xxx=false

parts/backgrounds/badapple.lua

@@ -51,6 +51,6 @@ function back.draw()
 	gc.pop()
 end
 function back.discard()
-	video=nil
+	video=false
 end
 return back

parts/backgrounds/fan.lua

@@ -70,6 +70,6 @@ function back.draw()
 	gc.pop()
 end
 function back.discard()
-	petal=nil
+	petal=false
 end
 return back

parts/backgrounds/space.lua

@@ -43,6 +43,6 @@ function back.draw()
 	gc.translate(10,10)
 end
 function back.discard()
-	stars=nil
+	stars=false
 end
 return back

parts/backgrounds/tunnel.lua

@@ -38,6 +38,6 @@ function back.draw()
 	end
 end
 function back.discard()
-	ring=nil
+	ring=false
 end
 return back

parts/backgrounds/wing.lua

@@ -68,6 +68,6 @@ function back.event(level)
 	end
 end
 function back.discard()
-	bar,crystal=nil
+	bar,crystal=false,false
 end
 return back