WIP: libgl-d3d12 icd installation

This one is meant mostly for @jenatali, as it touches on something we've been discussing.

I found an old patch from when I was testing the ICD-only bits a while back, so I cleaned it up slightly.

The idea is to generate a reg-file that will install the ICD. We could use add meson.add_install_script to automatically register the ICD when we do "ninja install"... The bit I don't like, is that this is a one-way install; we can't really reasonably store away the old register values I think and uninstall to those, can we?

Also, this is quite unfinished, as it doesn't fill in any of the values in a meaningful way. I also don't know if I even need all of these things. What does OpenGLVersion and OpenGLFlags even mean? 🤷

What do you think?