fixed a bug where the loading plaque caused front buffer rendering of all later stere...

fixed a bug where the loading plaque caused front buffer rendering of all later stereo views (glDrawBuffer was not being set in stereo rendering, except when vid_stereobuffer was used)
swapped left/right views so that positive values for r_stereo_separation produce normal stereo, not cross-eyed
added r_stereo_angle cvar based on a patch from Thomas Spriggs, this allows you to make the views look different directions, in addition to the existing offset capability

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@7057 d7cf8633-e32d-0410-b094-e92efae38249