Fix building with use_volk=yes on MacOS

[dsnopek]

Without the change in this PR, running scons platform=macos arch=arm64 dev_build=yes use_volk=yes I get these errors:

platform/macos/rendering_context_driver_vulkan_macos.mm:42:9: error: use of undeclared identifier 'VK_EXT_METAL_SURFACE_EXTENSION_NAME'
        return VK_EXT_METAL_SURFACE_EXTENSION_NAME;
               ^
platform/macos/rendering_context_driver_vulkan_macos.mm:48:2: error: unknown type name 'VkMetalSurfaceCreateInfoEXT'; did you mean 'VkHeadlessSurfaceCreateInfoEXT'?
        VkMetalSurfaceCreateInfoEXT create_info = {};
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~
        VkHeadlessSurfaceCreateInfoEXT
thirdparty/vulkan/include/vulkan/vulkan_core.h:14897:3: note: 'VkHeadlessSurfaceCreateInfoEXT' declared here
} VkHeadlessSurfaceCreateInfoEXT;
  ^
platform/macos/rendering_context_driver_vulkan_macos.mm:50:14: error: no member named 'pLayer' in 'VkHeadlessSurfaceCreateInfoEXT'
        create_info.pLayer = *wpd->layer_ptr;
        ~~~~~~~~~~~ ^
platform/macos/rendering_context_driver_vulkan_macos.mm:53:17: error: use of undeclared identifier 'vkCreateMetalSurfaceEXT'; did you mean 'vkCreateHeadlessSurfaceEXT'?
        VkResult err = vkCreateMetalSurfaceEXT(instance_get(), &create_info, nullptr, &vk_surface);
                       ^~~~~~~~~~~~~~~~~~~~~~~
                       vkCreateHeadlessSurfaceEXT
thirdparty/volk/volk.h:1350:39: note: 'vkCreateHeadlessSurfaceEXT' declared here
extern PFN_vkCreateHeadlessSurfaceEXT vkCreateHeadlessSurfaceEXT;

With the changes in this PR, compilation succeeds, and loading MoltenVK via Volk seems to work in my testing!

However, I'm not terribly knowledgeable of either MacOS or Vulkan, so I'm not sure if this is the correct fix, or I've got some issue on my system.

[bruvzg]

Have not tested it, but it probably can be set without if env["use_volk"]:, and for both iOS and macOS.

[dsnopek]

Ah, I haven't tested building for iOS, but looking at rendering_context_driver_vulkan_ios.mm, I think you're right that this is needed for iOS as well.

In my latest push, the define is set without if env["use_volk"] for both platforms.

EDIT: I just tested building for iOS with scons p=ios target=template_debug use_volk=yes and it built fine! I didn't try actually running it, though.

[dsnopek]

I added some other minor changes that appear to be necessary to actually run Godot normally when loading MoltenVK via Volk - there's a flag and extra extension needed.

(It was working in my testing before because I was running a test project using the Meta XR Simulator, which apparently doesn't care about this extension and flag. However, when running Godot without using the Meta XR Simulator, these extra changes are needed.)

Some docs about the flag and extension:

From https://docs.vulkan.org/tutorial/latest/03_Drawing_a_triangle/00_Setup/01_Instance.html#_encountered_vk_error_incompatible_driver

I'm not sure why this isn't needed when not loading via Volk, though.

[dsnopek]

I'm not sure why this isn't needed when not loading via Volk, though.

Looking into this a little more, I think I now understand why that extension isn't needed when MoltenVK is statically linked. From the MoltenVK docs:

That extension is only used by the Vulkan loader, and when MoltenVK is statically linked, there's no proper Vulkan loader involved.

However, since this PR is about getting Godot working with Volk (a Vulkan loader!), it is needed in this case. In fact, if we attempt to use that extension when not using Volk (ie when statically linking MoltenVK), then vkCreateInstance() will fail due to that extension not being supported.

So, I'm a little bit more confident about that change in this PR!

[akien-mga]

Thanks!