背景系统增加“默认背景”功能，新增节日飘雪背景

parts/backgrounds/snow.lua

@@ -0,0 +1,54 @@
+--Snow
+local gc=love.graphics
+local rnd=math.random
+local max,min=math.max,math.min
+local ins,rem=table.insert,table.remove
+local back={}
+
+local t
+local snow
+function back.init()
+	t=0
+	snow={}
+	BG.resize()
+end
+function back.update()
+	t=t+1
+	if t%(t%626>260 and 3 or 6)==0 then
+		ins(snow,{
+			x=SCR.w*rnd(),
+			y=0,
+			vy=1+rnd()*.6,
+			vx=rnd()*2-.5,
+			rx=2+rnd()*2,
+			ry=2+rnd()*2,
+		})
+	end
+	for i=#snow,1,-1 do
+		local P=snow[i]
+		P.y=P.y+P.vy
+		if P.y>SCR.h then
+			rem(snow,i)
+		else
+			P.x=P.x+P.vx
+			P.vx=P.vx-.02+rnd()*.04
+			P.rx=max(min(P.rx+rnd()-.5,4),2)
+			P.ry=max(min(P.ry+rnd()-.5,5),3)
+		end
+	end
+end
+function back.draw()
+	gc.clear(.2,.2,.2)
+	gc.push("transform")
+	gc.setColor(.7,.7,.7)
+	gc.origin()
+	for i=1,#snow do
+		local P=snow[i]
+		gc.ellipse("fill",P.x,P.y,P.rx,P.ry)
+	end
+	gc.pop()
+end
+function back.discard()
+	snow=false
+end
+return back