- make_oakengine_gtest embeds oakengine-obj; test code must see
OAKENGINE_API as dllexport too (OAKENGINE_BUILD), otherwise MinGW
resolves nothing against __imp_* stubs
- the render worker intentionally drops QT_QPA_PLATFORM=offscreen and
needs a real display for its GL context; run the Linux ctest and the
filtered gtest steps under xvfb-run