[MMc] Overnight GUI improvements

I made some changes last night, working from my laptop without a DK2. This verifies that you can use VRApp without the physical hardware installed (my brand-new laptop with Apple's idea of a "fast" GPU can only render at 6 fps, so you wouldn't want a DK2 attached).

I added m_vrHead as a MarkerEntity and m_vrEyeCamera[2] as Cameras to VRApp. These are in the scene--you can attach objects to them and query them as regular scene objects. This should make it easier for folks who are creating real-time body avatars (Mike and Dan).

I also added a mouse cursor the the in-VR HUD and a default dim black background for the screen so that the edges are obvious. The one problem with this is that when the mouse leaves the OSWindow, it simply appears stuck at its last position on the virtual screen and that's a little jarring.

The next, and potentially last (!), step: allow a virtual large monitor, so that we can have readable virtual displays with a large amount of text and controls on them. This will be the Iron Man-style body-space 2.5 display.

If I finish ahead of the deadline then I'd like to abstract all of the Oculus-specific code from VRApp into a HMD class (similar to OSWindow for VR) that can then be implemented for different HMDs. I'd also like to play with dynamic depth of field a little.