Fall-back to OpenGL renderer if mac hardware doesn't support Metal

webrtc/sdk/objc/Framework/Classes/Metal/RTCMTLNSVideoView.m

 /*
  *  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.
  */
 
@@ skipping 32 matching lines @@
   self = [super initWithCoder:aCoder];
   if (self) {
     [self configure];
   }
   return self;
 }
 
 #pragma mark - Private
 
 + (BOOL)isMetalAvailable {
-  return YES;
+  return [MTLCopyAllDevices() count] > 0;
 }
 
 - (void)configure {
   if ([[self class] isMetalAvailable]) {
     _metalView = [[MTKView alloc] initWithFrame:self.bounds];
     [self addSubview:_metalView];
     _metalView.layerContentsPlacement = NSViewLayerContentsPlacementScaleProportionallyToFit;
     _metalView.translatesAutoresizingMaskIntoConstraints = NO;
     _metalView.framebufferOnly = YES;
     _metalView.delegate = self;
@@ skipping 45 matching lines @@
 }
 
 - (void)renderFrame:(nullable RTCVideoFrame *)frame {
   if (frame == nil) {
     return;
   }
   self.videoFrame = [frame newI420VideoFrame];
 }
 
 @end