Cross vendor mesh shader support

Khronos finally announced the new mesh shader extension which aims to be comparable in terms of features with DirectX 12. So it would make sense to adjust the current mesh shading example project to use the new extension instead of Nvidias own extension, they released back then.

Here is a blog post about the differences to port the existing code and what needs to be addressed to optimize it as well, getting good performance across different GPUs and vendors.