Fix Shaders for stereo

[bretjackson]

Dan says: "Use of shaders to implement checkerboard and interlaced stereo seems like it might be problematic since having those shaders active when application::draw() is called will probably result in a conflict since most applications these days will also use shaders. Perhaps an alternative would be to set the rendering target to an offscreen FBO, call application::draw() to draw each eye to a separate FBO, and then combine the resulting images using a shader that is only active during this final compositing step while MinVR has complete control of the drawing."

I think these were never properly debugged any way and may not currently work.