Cocoa: Add fully dynamic loading of Vulkan loader

If the application is not linked against the Vulkan loader and relies on
a third-party loader library or glfwGetInstanceProcAddress, then our
call to dlopen will fail without a custom dyld environment variable.

This adds a fallback of looking in the directory of the main executable,
which matches the bundle structure recommended by the Vulkan SDK, making
that finally work out of the box for fully dynamic loading.

(cherry picked from commit 7da87aaae7)

src/cocoa_init.m

@@ -463,6 +463,32 @@ static GLFWbool initializeTIS(void)
 @end // GLFWApplicationDelegate
 
 
+//////////////////////////////////////////////////////////////////////////
+//////                       GLFW internal API                      //////
+//////////////////////////////////////////////////////////////////////////
+
+void* _glfwLoadLocalVulkanLoaderNS(void)
+{
+    CFBundleRef bundle = CFBundleGetMainBundle();
+    if (!bundle)
+        return NULL;
+
+    CFURLRef url =
+        CFBundleCopyAuxiliaryExecutableURL(bundle, CFSTR("libvulkan.1.dylib"));
+    if (!url)
+        return NULL;
+
+    char path[PATH_MAX];
+    void* handle = NULL;
+
+    if (CFURLGetFileSystemRepresentation(url, true, (UInt8*) path, sizeof(path) - 1))
+        handle = _glfw_dlopen(path);
+
+    CFRelease(url);
+    return handle;
+}
+
+
 //////////////////////////////////////////////////////////////////////////
 //////                       GLFW platform API                      //////
 //////////////////////////////////////////////////////////////////////////