No way to access instance from samples anymore

[SaschaWillems]

With #910 some public members were moved to private and can now only be accessed via functions, e.g. has_device() instead of device.

But it looks like there is no way to access the instance anymore. There is no has_instance(), but I need to access it for my shader debug printf sample like this:

	if (instance)
	{
		vkDestroyDebugUtilsMessengerEXT(instance->get_handle(), debug_utils_messenger, nullptr);
	}

[SaschaWillems]

As this is blocking me with #945 I'll try to fix in in that PR.