rearranged sky rendering code

rearranged sky rendering code
reenabled sky sphere code
added r_skyquality cvar (0 = no sky, higher values improve quality of quake sky rendering, no change to skybox)
sky sphere uses identical texture calculations as glquake style now
added r_mergesky cvar (speed loss in my testing, but could be useful for very low fillrate cards, albeit a bit jerky)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@217 d7cf8633-e32d-0410-b094-e92efae38249