Vulkan: Pass CAMetalLayer to MoltenVK instead of NSView

Gets rid of the warning for calling [NSView layer] off the main thread.

Source/Core/VideoBackends/Vulkan/main.cpp

@@ -342,6 +342,10 @@ void VideoBackend::PrepareWindow(WindowSystemInfo& wsi)
   // layer.contentsScale = factor
   reinterpret_cast<void (*)(id, SEL, double)>(objc_msgSend)(layer, sel_getUid("setContentsScale:"),
                                                             factor);
+
+  // Store the layer pointer, that way MoltenVK doesn't call [NSView layer] outside the main thread.
+  wsi.render_surface = layer;
+
   // The Metal version included with MacOS 10.13 and below does not support several features we
   // require. Furthermore, the drivers seem to choke on our shaders (mainly Intel). So, we warn
   // the user that this is an unsupported configuration, but permit them to continue.