Linking against MSVC 2019/2022 from a MingW compilation

[ggarra13]

I managed to use libplacebo to handle tonemapping on both Linux and macOS without any issues, by reading all the metadata of FFmpeg, following the discussion on #294.

Now my issue is on Windows. I am able to compile a libplacebo DLL on Windows with MingW64 to link against MSVC, which I thought it would fine as I thought libplacebo was all .c code. However, there seems to be at least one c++ file that relies on c++ std (convert.cc) and as such is not compatible. Well there's also glslang.cc but I am not using that.

Are there plans to change that C++ file with a C implementation? It seems all (I know it is quite a lot) needed to create pl_str_parse_* and pl_str_print_* for all the different types and make libplacebo a pure C library, instead of using template functions.