steam_helper: clean up OpenVR before getting OpenXR extensions in initialize_vr_data

When using OpenComposite, both OpenVR and OpenXR functions may
call the same underlying OpenXR loader.

Because the OpenXR loader only supports a single active instance,
`initialize_vr_data` currently fails as an OpenXR instance
has already been initialized ath the time XR extensions are queried.

This commit fixes the problem by cleaning up the temporary
OpenVR context *before* initializing OpenXR instead of keeping it
open until the end of the call.

Fix for:
https://github.com/ValveSoftware/Proton/issues/7905

steam_helper/steam.cpp

@@ -893,6 +893,11 @@ static DWORD WINAPI initialize_vr_data(void *arg)
         }
     }
 
+    if (vr_initialized) {
+        client_core->Cleanup();
+        vr_initialized = FALSE;
+    }
+
     if ((hwineopenxr = LoadLibraryA("wineopenxr.dll")))
     {
         p__wineopenxr_get_extensions_internal = reinterpret_cast<decltype(p__wineopenxr_get_extensions_internal)>