Add support for a checkerboard stereoscopic 3D mode for OpenGL

This patch adds support for a checkerboard 3D mode on OpenGL systems. 

It is basically a clone of the interlaced mode, just with a different stipple pattern. I have changed the name of the interlaced pattern to "stipple_interlaced" to avoid confusion. The rest of the patch should explain itself.

I don't know if the patch is complete as I don't have the possibility to test it right now. I suspect the 3D mode has to be enabled for the GUI to select it, but I couldn't find any clue.