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.

src/vulkan.c

@@ -57,6 +57,8 @@ GLFWbool _glfwInitVulkan(int mode)
     _glfw.vk.handle = _glfw_dlopen("vulkan-1.dll");
 #elif defined(_GLFW_COCOA)
     _glfw.vk.handle = _glfw_dlopen("libvulkan.1.dylib");
+    if (!_glfw.vk.handle)
+        _glfw.vk.handle = _glfwLoadLocalVulkanLoaderNS();
 #else
     _glfw.vk.handle = _glfw_dlopen("libvulkan.so.1");
 #endif