Introduced a new Metal renderer implementation and a new example illustrating usage of Metal on macOS and iOS (partially addresses #1873)

examples/example_apple_metal/Shared/AppDelegate.h

@@ -0,0 +1,19 @@
+
+#import <TargetConditionals.h>
+
+#if TARGET_OS_IPHONE
+
+#import <UIKit/UIKit.h>
+
+@interface AppDelegate : UIResponder <UIApplicationDelegate>
+@property (strong, nonatomic) UIWindow *window;
+@end
+
+#else
+
+#import <Cocoa/Cocoa.h>
+
+@interface AppDelegate : NSObject <NSApplicationDelegate>
+@end
+
+#endif