iOS: Add interface for injecting custom shaders

webrtc/sdk/objc/Framework/Classes/Video/RTCDefaultShader.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;
+#import "WebRTC/RTCVideoViewShading.h"
 
 NS_ASSUME_NONNULL_BEGIN
 
-@interface RTCNV12TextureCache : NSObject
-
-@property(nonatomic, readonly) GLuint yTexture;
-@property(nonatomic, readonly) GLuint uvTexture;
-
-- (nullable instancetype)initWithContext:(EAGLContext *)context;
-
-- (BOOL)uploadFrameToTextures:(RTCVideoFrame *)frame;
-
-- (void)releaseTextures;
+/** Default RTCVideoViewShading that will be used in RTCNSGLVideoView and
+ *  RTCEAGLVideoView if no external shader is specified. This shader will render
+ *  the video in a rectangle without any color or geometric transformations.
+ */
+@interface RTCDefaultShader : NSObject<RTCVideoViewShading>
 
 @end
 
 NS_ASSUME_NONNULL_END