Possible issue with path separators

[javagl]

There has been an issue report, indicating that the output of cdb-to-3dtiles cannot be loaded in CesiumJS. According to the thread at https://community.cesium.com/t/error-on-b3dm-from-3dtiles/31944/5 , CesiumJS could not load a file that was named like Textures\example.jpeg, and it had to be fixed somewhere here in CDBTo3DTiles.cpp.

(I assume that a std::filesystem::path will be converted into the platform-specific string, which may include a \ on windows - but for a URI, it should always be a /)