Generate bindings from the C++ header instead of C

[Ralith]

The official C bindings are becoming steadily more broken as time goes on. As it appears that the C++ API is the only reliable target we can feasibly interact with, switching to them going forwards may reduce maintenance load. This would however be a massively breaking change, which may not be justified if OpenXR releases any time soon.

[Ybalrid]

This needs to be re-evaluated... Many years have passed! OpenVR_CAPI.h should be quite stable. I fugre the C# bindings actually call the symbols defined here (not sure about that, vague memory from when debugging Unity apps some time ago).