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

BUG=webrtc:7782

Review-Url: https://codereview.webrtc.org/2927983002
Cr-Commit-Position: refs/heads/master@{#18539}

webrtc/examples/objc/AppRTCMobile/mac/APPRTCViewController.m

@@ -221,10 +221,12 @@ static NSUInteger const kBottomViewHeight = 200;
 // If not we're providing sensible default.
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wpartial-availability"
-  if ([RTCMTLNSVideoView class]) {
+  if ([RTCMTLNSVideoView class] && [RTCMTLNSVideoView isMetalAvailable]) {
     _remoteVideoView = [[RTCMTLNSVideoView alloc] initWithFrame:NSZeroRect];
     _localVideoView = [[RTCMTLNSVideoView alloc] initWithFrame:NSZeroRect];
-  } else {
+  }
+#pragma clang diagnostic pop
+  if (_remoteVideoView == nil) {
     NSOpenGLPixelFormatAttribute attributes[] = {
       NSOpenGLPFADoubleBuffer,
       NSOpenGLPFADepthSize, 24,
@@ -245,7 +247,6 @@ static NSUInteger const kBottomViewHeight = 200;
     local.delegate = self;
     _localVideoView = local;
   }
-#pragma clang diagnostic pop
 
   [_remoteVideoView setTranslatesAutoresizingMaskIntoConstraints:NO];
   [self addSubview:_remoteVideoView];

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

@@ -50,7 +50,7 @@
 #pragma mark - Private
 
 + (BOOL)isMetalAvailable {
-  return YES;
+  return [MTLCopyAllDevices() count] > 0;
 }
 
 - (void)configure {

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

@@ -13,5 +13,5 @@
 
 NS_AVAILABLE_MAC(10.11)
 @interface RTCMTLNSVideoView : NSView<RTCVideoRenderer>
-
++ (BOOL)isMetalAvailable;
 @end