* Added SBSLETTERBOX to menudef.txt
* Changed PresentSideBySide in gl_renderer
Needed for new side-by-side mode
* Added VR_SIDEBYSIDELETTERBOX to gl_stereo3d.cpp
For side by side letterbox display mode
* Added VR_SIDEBYSIDELETTERBOX to hw_modes.h
For new side by side letterbox display mode
* Added VR_SIDEBYSIDELETTERBOX to hw_vrmodes.cpp
For Side By Side Letterbox display mode