add
Register a needCallback to be invoked when a new video frame has been received. This version。 uses the drawer of the EglRenderer that was passed in init.
addFrameListener方法调用必须在打开摄像头、设置 RCRTCVideoView ，通话建立成功之后才能获取到非空的 bitmap ;
addFrameListener方法是调用一次返回一帧图像的 bitmap 。必须在 结束通话 或者 使用完之后注销监听;
Parameters
listener
The needCallback to be invoked.
scale
The scale of the Bitmap passed to the needCallback, or 0 if no Bitmap is required.