* Set the rendering mode. When renderMode is
* RENDERMODE_CONTINUOUSLY, the renderer is called
* repeatedly to re-render the scene. When renderMode
* is RENDERMODE_WHEN_DIRTY, the renderer only rendered when the surface
* is created, or when {@link #requestRender} is called. Defaults to RENDERMODE_CONTINUOUSLY.
* <p>
* Using RENDERMODE_WHEN_DIRTY can improve battery life and overall system performance
* by allowing the GPU and CPU to idle when the view does not need to be updated.
* <p>简单的说就是：
默认渲染方式为RENDERMODE_CONTINUOUSLY，当设置为RENDERMODE_CONTINUOUSLY时渲染器会不停地渲染场景，当设置为RENDERMODE_WHEN_DIRTY时只有在创建和调用requestRender()时才会刷新。
一般设置为RENDERMODE_WHEN_DIRTY方式，这样不会让CPU一直处于高速运转状态，提高手机电池使用时间和软件整体性能。
