Add metal view, shaders and renderer.

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMTLRenderer.h

+/*
+ *  Copyright 2016 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/RTCVideoRenderer.h"
+
+@class MTKView;
+
+NS_ASSUME_NONNULL_BEGIN
+RTC_EXPORT
+/**
+ * Render class that uses Metal to render RTCVideoFrame in an instance of MTKView.
+ */
+@protocol RTCMTLRenderer<RTCVideoRenderer>
+
+- (instancetype)initWithView:(__weak __kindof MTKView *)view;
+
+/**
+ * Sets up the renderer.
+ *
+ * If the method returns NO it signifies the renderer is unable to perform proper setup and
+ * it's responsibility of the caller to perform all necessary clean up after this point.
+ * Unexpexted behavior can happen if the cleanup doesn't happen and the renderer continues
+ * accepting RTCVideoRenderer callbacks.
+ */
+- (BOOL)setup;
+
+@end
+
+NS_ASSUME_NONNULL_END