virgil3d: design document and sorta build instructions

I've published on Google docs a bit more of a technical document on how virgil3d is designed.

https://docs.google.com/document/d/1cL2sEi5DAd-qY_OzyNiUyIbcU2ZvcI8lhv3PWzFoH_w/pub

I'm hoping to flesh it out a bit more, and of course I'll probably never keep it up to date, but it should be close enough :-)

I've also put up some build instructions here:
https://docs.google.com/document/d/1CNiN0rHdfh7cp9tQ3coebNEJtHJzm4OCWvF3qL4nucc/pub

They are messy and incomplete, any don't go packaging anything.