Fix Vulkan resource cleanup in hello_xr

[HussainAther]

Summary

Add explicit cleanup for Vulkan resources owned by VulkanGraphicsPlugin.

The destructor now:

• waits for the Vulkan device to become idle
• releases swapchain image and rendering resources
• destroys the draw semaphore
• resets the memory allocator
• destroys the Vulkan device
• destroys the debug messenger
• destroys the Vulkan instance
• clears the associated Vulkan handles

This prevents hello_xr from leaking its Vulkan device and instance and allows validation layers to report resource leaks more accurately.

Fixes #211.

Testing

• cmake --build build
• ctest --test-dir build

Result:

100% tests passed, 0 tests failed out of 1

[CLAassistant]

All committers have signed the CLA.

[rpavlik-bot]

An issue (number 2785) has been filed to correspond to this pull request in the internal Khronos GitLab (Khronos members only: KHR:openxr/openxr#2785 ), to facilitate working group processes.

This GitHub pull request will continue to be the main site of discussion.

[johnkearney]

This looks like a good change to me but as I compare it to the equivalent code in the conformance test suite I'm seeing divergence. Generally we've tried to keep these inline - if there's a way to keep them more similar that'd be great. But generally LGTM.