Implement first 3 stereo 3D modes: Mono, Green/Magenta, and Side by Side.
Create several global variables related to 3D, including some console variables.
Merge branch 'oculus-rift' of https://github.com/cmbruns/Doomsday-Engine into oculus-rift

Conflicts:
	doomsday/client/src/ui/clientwindow.cpp
ClientWindow: Update logical UI size when VR mode changes