Cocoa: Add basic support for Vulkan via MoltenVK

This adds basic support for MoltenVK, a Vulkan implementation on top of
Metal, on macOS 10.11 and later.  It looks for MoltenVK in the process
via RTLD_DEFAULT symbol lookup if _GLFW_VULKAN_STATIC is disabled.

glfwCreateWindowSurface now creates and sets a CAMetalLayer for the
window content view, which is required for MoltenVK to function.

You must help CMake find MoltenVK for the Vulkan test to be built.

Fixes #870.

src/cocoa_platform.h

@@ -39,6 +39,18 @@
 typedef void* id;
 #endif
 
+typedef VkFlags VkMacOSSurfaceCreateFlagsMVK;
+
+typedef struct VkMacOSSurfaceCreateInfoMVK
+{
+    VkStructureType                 sType;
+    const void*                     pNext;
+    VkMacOSSurfaceCreateFlagsMVK    flags;
+    const void*                     pView;
+} VkMacOSSurfaceCreateInfoMVK;
+
+typedef VkResult (APIENTRY *PFN_vkCreateMacOSSurfaceMVK)(VkInstance,const VkMacOSSurfaceCreateInfoMVK*,const VkAllocationCallbacks*,VkSurfaceKHR*);
+
 #include "posix_tls.h"
 #include "cocoa_joystick.h"
 #include "nsgl_context.h"
@@ -74,6 +86,7 @@ typedef struct _GLFWwindowNS
     id              object;
     id              delegate;
     id              view;
+    id              layer;
 
     GLFWbool        maximized;