ci: declare all oakengine handle types opaque for Qt metatypes; defer gtest discovery on Windows

- Linux GCC rejects QMetaType instantiation over incomplete types
  (static_assert(sizeof(T))); Q_DECLARE_OPAQUE_POINTER every opaque
  OakEngine* handle in the public headers so QList<OakEngineTask*> etc
  compile
- Windows: gtest discovery ran the fresh exe before the DLLs were next
  to it; use DISCOVERY_MODE PRE_TEST on WIN32 and copy the DLLs for
  ctest time

engine/include/oakengine/preview.h

@@ -246,6 +246,7 @@ OAKENGINE_API void oakengine_preview_request_free(
 
 #ifdef __cplusplus
 }
+Q_DECLARE_OPAQUE_POINTER(OakEnginePreviewRequest *)
 #endif
 
 #endif /* OAKENGINE_PREVIEW_H */