mingw-w64 compatibility for the C++ API

[CesarBerriot]

Hi!
First of all I wish to thank the entire Ultralight team for all the amazing work.

I'm making this issue to reopen the discussion of mingw-w64 compatibility for the C++ API.
It's been discussed and concluded here in august 2019, now over 5 years ago, over which I imagine a lot changed on your side.

I'm no C++ enthusiast, and personally use the C API for desktop software development, but I figured this theoretically shouldn't be much of a hassle considering Ultralight builds flawlessly under GCC on linux already.

This would bring wider compatibility to ultralight, and facilitate its integration within linux/windows cross-platform applications which rely on GCC to handle compilation.

From my understanding the current incompatibility is due to differences between MSVC and GCC's name mangling algorithms.