Vulkan: Support macOS via MoltenVK

The path to the MoltenVK library can be specified by the
LIBMOLTENVK_PATH environment variable, otherwise it assumes it is
located in the application bundle's Contents/MacOS directory.

Source/Core/VideoBackends/Vulkan/VideoBackend.h

@@ -18,5 +18,6 @@ public:
   std::string GetName() const override { return "Vulkan"; }
   std::string GetDisplayName() const override { return _trans("Vulkan"); }
   void InitBackendInfo() override;
+  void PrepareWindow(const WindowSystemInfo& wsi) override;
 };
 }  // namespace Vulkan