3D component in process: expect some turbulence ahead

[Randall O'Reilly]

I’ve added the [glfw](https://github.com/go-gl/glfw) cross-platform GUI infrastructure system as the one currently-supported driver backend in oswin/drivers/glos, in order to support the new 3D scenegraph system within goki (in the gi3d sub-package).

There is now a nice oswin/gpu package that abstracts the GPU functionality in a way that should be portable for a future version using Vulkan instead of OpenGL, and the backend 2D rendering is now using this framework.

I’ve just pushed this new system in the main repository, to get it working on Linux / X11 and Windows — unlikely to work on those platforms until later tonight…

- Randy

[Randall O'Reilly]

My basic tests show everything working on all three platforms at this point (and a bonus that wayland now works). Most of the subsequent turbulence should be confined to the gi3d package and the rest of the 2D framework should remain usable and stable. I still have debug mode enabled on opengl so it might be a bit slower than it will eventually be once everything is working (OpenGL is used in 2D for the basic blitting of the rendered window image to actually show up on the monitor).

Also note that there are new installation requirements on windows and linux due to the use of cgo on those platforms. I will also eventually reinstate the previous 2D-only backends for those, which do not depend on cgo, but that is relatively lowpri for me at this point. https://github.com/goki/gi/issues/266

- Randy