重构背景系统（每个背景使用独立文件），全局变量名整理

parts/backgrounds/lightning2.lua

@@ -0,0 +1,27 @@
+--Fast lightning + spining tetromino
+local gc=love.graphics
+local int,rnd=math.floor,math.random
+local back={}
+
+local t
+local colorLib
+local blocks=BLOCKS
+local scs=spinCenters
+function back.init()
+	colorLib=SKIN.libColor
+	t=rnd()*2600
+end
+function back.update(dt)
+	t=t+dt
+end
+function back.draw()
+	local R=7-int(t*.5%7)
+	local T=1.2-t%10%3%1.2
+	if T<.3 then gc.clear(T,T,T)
+	else gc.clear(0,0,0)
+	end
+	local _=colorLib[SETTING.skin[R]]
+	gc.setColor(_[1],_[2],_[3],.12)
+	gc.draw(TEXTURE.miniBlock[R],640,360,t%3.1416*6,400,400,scs[R][0][2]+.5,#blocks[R][0]-scs[R][0][1]-.5)
+end
+return back