[MMc] First successful HUD rendering in VR

Pressing the Tab key now moves the G3D GUI (everything in onGraphics2D) from the debugging mirror display into the VR world. It appears in front of the HMD, floating about 1.5m away from the viewer. The effect is surprisingly cool even with a boring debugging GUI when you're in the HMD--it is augmented reality for a virtual reality world.

Just press Tab to toggle this on and off in any VRApp.


I've succeeded in all of my planned tasks today by upgrading VRApp to give better performance and convenience and then rendering the G3D GUI into the HMD.

There are some problems with the GUI view that I'll tackle tomorrow:

1. The HUD needs to be low resolution because the DK2 itself is low resolution. This currently forces the developer to make the on-screen window low resolution as well so that everything is consistent for event delivery. That's annoying because it means that debugging mirror mode is only at 640x400, and is hard to use. I welcome suggestions for how to deal with this. Maybe I need to make a virtual desktop in body space instead of a layer in head space.
2. There is no cursor in the HMD (should be easy to handle)
3. The boundaries of the in-HMD screen aren't clear (should be easy to handle), making it confusing when a window is cropped
4. The GUI is rendered twice. Once for the HMD and once on the main screen. I should be able to just copy the HMD's texture to the main screen, but there's a bug in that code right now.