Add Vulkan graphics backend on Windows (currently crashes at xrCreateSession)

[dfattal]

Summary

Switching Windows graphics API to Vulkan (m_APIs: 15000000 first in ProjectSettings.asset) causes the app to crash inside xrCreateSession. The plugin's session-creation code at native~/displayxr_hooks.cpp:722-741 has no Vulkan branch on Windows:

#if defined(_WIN32)
    if (item->type == XR_TYPE_GRAPHICS_BINDING_D3D12_KHR) { ...d3d12... }
    else if (item->type == XR_TYPE_GRAPHICS_BINDING_D3D11_KHR) { ...d3d11... }
#elif defined(__APPLE__)
    if (item->type == ...METAL_KHR) { ...metal... }
#endif

When Unity uses Vulkan, s_backend = nullptr and downstream code crashes. The log line at native~/displayxr_hooks.cpp:710 is also misleading:

[DisplayXR] Graphics binding: VULKAN (via MoltenVK on macOS)

— fires unconditionally, suggesting the original author assumed Vulkan only ever came from macOS + MoltenVK.

Crash signature

displayxr_unity!<unknown>
UnityOpenXR!session_RequestExitSession
UnityEngine.XR.OpenXR.OpenXRLoaderBase:Internal_CreateSessionIfNeeded
UnityEngine.XR.OpenXR.OpenXRLoaderBase:StartInternal

Why it matters

Discovered while investigating XR eye-texture MSAA on Windows (companion issue: "Build-path silhouette aliasing"). Unity OpenXR on Vulkan historically has different MSAA handling than D3D12 — Vulkan on Windows is a plausible workaround path for the silhouette aliasing problem if the plugin supported it. Currently can't even test the hypothesis.

Scope

Add a Windows-capable Vulkan backend mirroring the structure of native~/displayxr_d3d12_backend.cpp. The existing native~/displayxr_vulkan_backend.cpp (currently macOS-only via MoltenVK) and native~/displayxr_standalone_vulkan.cpp (SA-session Vulkan path) likely have reusable code.

Also fix the misleading log message to reflect the actual platform.

Acceptance

• App doesn't crash with m_APIs: 150000001200000002000000 (Vulkan first, D3D12/D3D11 fallback) in test-transparent's ProjectSettings.asset.
• displayxr.log shows a correct Vulkan binding line.
• Ghost asset renders without geometric corruption.
• Bonus: MSAA probe in TransparentAutoSetup shows whether Vulkan unlocks XR.eye.msaa > 1 (the original question we wanted to answer).