Examples: Apple: add Makefile for example_apple_metal, example_apple_opengl2. (#8637)

- Works only for macOS: use XCode for iOS
- It will produce a raw exe. It will not a produce an macOS application bundle (i.e a folder that includes exe and resources). To get app bundles, use the XCode project.
- An adjustment was required in main.mm (for macOS only), to ensure that the exe can run without any plist or storyboard under macOS

examples/example_apple_metal/main.mm

@@ -319,9 +319,20 @@
 
 #if TARGET_OS_OSX
 
-int main(int argc, const char * argv[])
+int main(int, const char**)
 {
-    return NSApplicationMain(argc, argv);
+    @autoreleasepool
+    {
+        [NSApplication sharedApplication];
+        [NSApp setActivationPolicy:NSApplicationActivationPolicyRegular];
+
+        AppDelegate *appDelegate = [[AppDelegate alloc] init];   // creates window
+        [NSApp setDelegate:appDelegate];
+
+        [NSApp activateIgnoringOtherApps:YES];
+        [NSApp run];
+    }
+    return 0;
 }
 
 #else