Instantiate the CAMetalLayer so SDL_Vulkan_CreateSurface() doesn't fail

Error message was:
[mvk-info] MoltenVK version 0.18.2. Vulkan version 1.0.51.
[***MoltenVK ERROR***] VK_ERROR_INITIALIZATION_FAILED: On-screen rendering requires a view that is backed by a layer of type CAMetalLayer.
2017-08-28 02:17:29.579 testvulkan[95627:1716939] ERROR: SDL_Vulkan_CreateSurface(): vkCreateMacOSSurfaceMVK failed: VK_ERROR_INITIALIZATION_FAILED

src/video/cocoa/SDL_cocoametalview.m

@@ -94,8 +94,9 @@ Cocoa_Mtl_AddMetalView(SDL_Window* window)
     SDL_cocoametalview *metalview
         = [[SDL_cocoametalview alloc] initWithFrame:view.frame
                        useHighDPI:(window->flags & SDL_WINDOW_ALLOW_HIGHDPI)];
+    // Instantiate the CAMetalLayer
+    metalview.wantsLayer = YES;
     [view addSubview:metalview];
-  
     return metalview;
 }