Correctly deinitialize wxUxThemeEngine in wxMSW.

Set ms_isThemeEngineAvailable to -1, meaning that we need to reinitialize
themes, instead of false, meaning that themes are not available.

This fixes problems when the library is initialized, shut down and then
initialized again.

Closes #17023.

src/msw/uxtheme.cpp

@@ -55,7 +55,7 @@ public:
             // something like this
             wxUxThemeEngine *themeEngine = wxUxThemeEngine::ms_themeEngine;
             wxUxThemeEngine::ms_themeEngine = NULL;
-            wxUxThemeEngine::ms_isThemeEngineAvailable = false;
+            wxUxThemeEngine::ms_isThemeEngineAvailable = -1;
 
             delete themeEngine;
         }