Add metal view, shaders and renderer.

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMTLRenderer.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 <Foundation/Foundation.h>
+#import "WebRTC/RTCMTLVideoView.h"
+
+NS_ASSUME_NONNULL_BEGIN
+RTC_EXPORT
+
+/**
+ * Protocol defining ability to render RTCVideoFrame in Metal enabled views.
+ */
+@protocol RTCMTLRenderer <RTCVideoRenderer>
+
+/**
+ * Method to be implemented to perform actual rendering of RTCVideoFrames.
+ *
+ * Implementors are responsbile for syncing execution of _draw_ and _renderFrame_ calls if needed.
+ */
+- (void)draw;
+@end
+
+NS_ASSUME_NONNULL_END