跑马灯：滚动公告的无缝循环

-- 错误：滚出边界就重置，留白一大段
function onTick(dt)
    x = x - SPEED * dt
    if x < -textW then
        x = W
    end
end

local SPEED, W = 80, 400
local textW = label:getContentSize().width
local off = 0
local function onTick(dt)
    off = (off + SPEED * dt) % textW
    label:setPosition(W - off, 0)
end