Chapter 1: Vulkan Init Code - member `_FrameNumber` not mentioned anywhere in modern guide.

in `VulkanEngine` class:

```c++
FrameData _frames[FRAME_OVERLAP];
FrameData& get_current_frame() { return _frames[_frameNumber % FRAME_OVERLAP]; };
```

probably it's mentioned in the legacy guide somewhere, but in the modern guide there is no declaration of this member in the code.