VulkanRenderer: implement metal view

The metal view backing the MoltenVK compatibility wrapper is written in
Obj-C.

It would have been possible to use the Objective-C runtime in C++ but
the code is less performant (the symbol binding is done at first use
instead of the beginning of the program) and actually harder to get
right.

Note that MoltenVK is not linked to the loader, so the libMoltenVK.dylib
object must be available for loading.

include/Nazara/VulkanRenderer/Wrapper/Instance.hpp

@@ -34,6 +34,10 @@
 #include <vulkan/vulkan_xcb.h>
 #endif
 
+#ifdef VK_USE_PLATFORM_METAL_EXT
+#include <vulkan/vulkan_metal.h>
+#endif
+
 #ifdef VK_USE_PLATFORM_XLIB_KHR
 typedef struct _XDisplay Display;
 typedef unsigned long XID;