原理：

1. 先在RenderTexture上渲染文字

2. 通过RenderTexture:newImage来获取到Image并依此得到对应的Texture

3 通过Texture构造Sprite，将生成的Sprite作为ClippingNode的模板(stencil)

4. ClippingNode的底图可以随意放置，这样我们就可以还不错的文字效果啦：）

-- 添加特效文本
-- example: addEffectText(self,300,400,"大家好",30,"1.png"); 
function addEffectText(parent,x,y,str,fontSize,bg)
    local target = cc.RenderTexture:create(640,50);   
    target:retain();
    target:setPosition(100,100);

    local text = cc.Label:createWithTTF(str,"fonts/simhei.ttf",fontSize);
    text:setColor(cc.c3b(255,0));
    text:setPosition(320,25);

    target:begin();
    text:visit();
    target:endToLua();
   
    local function copyImage()
        local pImage = target:newImage();

        local tex = cc.Director:getInstance():getTextureCache():addImage(pImage,str);

        local sprite = cc.Sprite:createWithTexture(tex);
        local clippingNode = cc.ClippingNode:create();
        local image = cc.Sprite:create(bg);
        clippingNode:setStencil(sprite);
        clippingNode:addChild(image);
        clippingNode:setAlphaThreshold(0.1);
        clippingNode:setPosition(x,y);
        parent:addChild(clippingNode);
        
        target:release();
    end
    
    performWithDelay(parent,copyImage,0.01);
end

效果：