Agora RTC Objective-C API Reference Refactor

Here is a list of all struct and union fields with links to the structures/unions they belong to:

### - y -

- y : MixerLayoutConfig , Rectangle , RtcImage , TranscodingUser , TranscodingVideoStream , VideoCompositingLayout::Region , AgoraFacePositionInfo
- yBuffer : VideoFrame , AgoraExtVideoFrame , AgoraOutputVideoFrame
- yRatio : WatermarkRatio
- yStride : VideoFrame , AgoraExtVideoFrame , AgoraOutputVideoFrame