Add metal view, shaders and renderer.

webrtc/sdk/objc/Framework/Classes/Metal/RTCNV12Renderer.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 <MetalKit/MTKView.h>
+
+#import "WebRTC/RTCMTLRenderer.h"
+
+/**
+ * Implementation of RTCMTLRenderer protocol for rendering native nv12 video frames.
+ */
+@interface RTCNV12Renderer : NSObject <RTCMTLRenderer>

[tkchin_webrtc, 2017/02/16 23:28:04]

nit: RTCMTLNV12Renderer or RTCMetalNV12Renderer
So that we can have a RTCOpenGLNV12Renderer or something

per other comment about shader interface though, can we hide this file for now
pending final interface design? Or otherwise just fold it in.

[daniela-webrtc, 2017/02/20 15:17:07]

Done.

+
+/**
+ * Sets the provided view as rendering destination if possible.
+ *
+ * If not possible method returns NO and callers of the method are responisble for performing
+ * cleanups.
+ */
+- (BOOL)addRenderingDestination:(__kindof MTKView *)view;
+@end