OStream mutex addition breaks managed code (C++/CLI) compilations.

[BStringhamVRSK]

Discussed in #5329

^{Originally posted by BStringhamVRSK March 17, 2022}
@pixelflinger @prideout @bejado
We are using Filament in a C# managed code environment. std::mutex is not supported when C++ code is compiled with the /clr flag. Because the std::mutex declaration appears in the include header file, it breaks our builds.

There are two ways forward that I can see:

1. Move the mutex into ostream.cpp as a global. That means all ostream instances will share the same mutex. PROS: This is probably the simplest change. CONS: This could have some impact on performance, but I don't know if it's a meaningful difference.
2. Create an interface for ostream. That would require also creating a "factory" function to create ostream instances, rather than using the new operator. PROS: ostream would continue to work as before. CONS: Breaks any code currently using "new ostream()"

Any other thoughts or suggestions on this?
I can do the programming changes, but I'd like to hear the code owners' opinions before diving in.

[Mathias Agopian]

I think maybe another solution would be to use a pImpl pattern. This would be fine for ostream as it's not performance critical. All the private gunk could go into the cpp.

[Mathias Agopian]

let us know how it goes.