Allows running more tests with nulldrv, including some D3D tests with vulkan renderer.
-- v3: win32u: Use VK_EXT_headless_surface for nulldrv surface. win32u: Pass a vulkan_instance pointer to vulkan_surface_create. winevulkan: Enable the VK_EXT_headless_surface extension. winevulkan: Generate function pointers for required funcs.