HelloXR Vulkan version enables USE_MIRROR_WINDOW default, and it is annoying when testing it on Windows

See https://github.com/KhronosGroup/OpenXR-SDK-Source/blob/main/src/tests/hello_xr/graphicsplugin_vulkan.cpp#L21.

From the comment, `USE_MIRROR_WINDOW` shows a dummy window when running Vulkan version of HelloXR on Windows platform, and this window is used for RenderDoc. As a normal tester, I don't use RenderDoc, and I just want the Runtime to display a native window to show the rendered content. What about disabling it default? If other folks want to run it with RenderDoc, they can build it with patch to enable `USE_MIRROR_WINDOW` macro.

Start at src/tests/hello_xr/graphicsplugin_vulkan.cpp near the USE_MIRROR_WINDOW definition and read its comment and surrounding Windows-specific setup. Verify how the Vulkan HelloXR path behaves with the macro disabled, then confirm that the default no longer opens the dummy RenderDoc window while the rendered content remains available in the native window.

Written by the indexing model from the issue text.