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/main.m

@@ -0,0 +1,23 @@
+
+#import <TargetConditionals.h>
+
+#if TARGET_OS_IPHONE
+
+#import <UIKit/UIKit.h>
+#import "AppDelegate.h"
+
+int main(int argc, char * argv[]) {
+    @autoreleasepool {
+        return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));
+    }
+}
+
+#else
+
+#import <Cocoa/Cocoa.h>
+
+int main(int argc, const char * argv[]) {
+    return NSApplicationMain(argc, argv);
+}
+
+#endif