iOS: Add interface for injecting custom shaders

webrtc/sdk/objc/Framework/Classes/Video/RTCNV12TextureCache.h

 /*
  *  Copyright 2017 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #import <GLKit/GLKit.h>
 
 @class RTCVideoFrame;
 
 NS_ASSUME_NONNULL_BEGIN
 
 @interface RTCNV12TextureCache : NSObject
 
 @property(nonatomic, readonly) GLuint yTexture;
 @property(nonatomic, readonly) GLuint uvTexture;
 
-- (nullable instancetype)initWithContext:(EAGLContext *)context;
+- (instancetype)init NS_UNAVAILABLE;
+- (nullable instancetype)initWithContext:(EAGLContext *)context NS_DESIGNATED_INITIALIZER;
 
 - (BOOL)uploadFrameToTextures:(RTCVideoFrame *)frame;
 
 - (void)releaseTextures;
 
 @end
 
 NS_ASSUME_NONNULL_END