经验条充能：升级跨段的分段动画

-- 错误：一次性写入总值，跨段直接跳变
function gainExp(amount)
    bar:setPercent((cur + amount) / need)
end

local function gainExp(amount, need, cur)
    local total = cur + amount
    if total < need then
        animateBar(cur, total)
        return
    end
    animateBar(cur, need, function()
        flashLevelUp()
        animateBar(0, total - need)
    end)
end