Fix some robustness issues in the examples

[aleino-nv]

Do some robustness fixes for a few of the examples.
This fixes a hang in the raytracing samples, and fixes the test failure in the hello-world Windows/Debug config.

This helps to address #5520.

[csyonghe]

Why is this needed?

[aleino-nv]

I check this value in HelloWorldExample::~HelloWorldExample which previously called some uninitialized VK API functions in the case where VK API initialization failed.

[csyonghe]

Uninitialized vk api should be nullptr, you should be able to check for the function pointer before calling it.

[aleino-nv]

Sure, but then we would need to check every function pointer.
This variable means that all of the pointers should be initialized so we can just check this one value.

[csyonghe]

I still don’t understand why this is needed. If the basic Vulkan functions are not found, shouldn’t we exit already? The VkAPI class already provides some methods that returns whether or not it is successfully loaded, no?

[csyonghe]

If it is the destructor that is causing the problem, checking for null vk device handle before calling destroy should be more than enough. If device is null, just return immediately.

[aleino-nv]

I still don’t understand why this is needed. If the basic Vulkan functions are not found, shouldn’t we exit already?

The destructor of the app still runs, regardless of whether initializeVulkanDevice succeeded.

On the other hand, I believe initializeVulkanDevice could successfully set some of the VK API pointers (but not others) if it doesn't make it all the way to the end and returns 0, so I don't think it's enough to just check one of the pointers.

[aleino-nv]

Here is what the current code looks like

int main(int argc, char* argv[])
{
    initDebugCallback();
    HelloWorldExample example;
    example.parseOption(argc, argv);
    return example.run(); // ...destructor still runs and uses VK API
}

int HelloWorldExample::run()
{
    RETURN_ON_FAIL(initVulkanInstanceAndDevice()); // <--- This can fail if VK API initialization fails
    RETURN_ON_FAIL(createComputePipelineFromShader());
    RETURN_ON_FAIL(createInOutBuffers());
    RETURN_ON_FAIL(dispatchCompute());
    RETURN_ON_FAIL(printComputeResults());
    return 0;
}

[aleino-nv]

The VkAPI class already provides some methods that returns whether or not it is successfully loaded, no?

Not as far as I can tell. Do you want me to move the boolean into the VkAPI struct and set it to true just before returning 0?

[aleino-nv]

If it is the destructor that is causing the problem, checking for null vk device handle before calling destroy should be more than enough. If device is null, just return immediately.

Ok I did something like this, please have a look.

[csyonghe]

always prefer early return over nesting if:

if (!vkAPI.device)
   return;

[aleino-nv]

Done.

[csyonghe]

Looks good!