对话打字机：逐字出现的对话文本

-- 错误：整段一次显示，节奏全无
function showDialog(text)
    label:setString(text)
end

local shown, STEP = 0, 3
local function onTick()
    if shown >= #fullText then return end
    shown = math.min(shown + STEP, #fullText)
    label:setString(fullText:sub(1, shown))
end
local function onSkip()
    shown = #fullText
    label:setString(fullText)
end