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/ViewController.h

@@ -0,0 +1,20 @@
+
+#import <Metal/Metal.h>
+#import <MetalKit/MetalKit.h>
+#import "Renderer.h"
+
+#if TARGET_OS_IPHONE
+
+#import <UIKit/UIKit.h>
+
+@interface ViewController : UIViewController
+@end
+
+#else
+
+#import <Cocoa/Cocoa.h>
+
+@interface ViewController : NSViewController
+@end
+
+#endif