implemented fog height setting, along with fade depth setting - global

implemented fog height setting, along with fade depth setting - global
fog now has a ceiling and a gradient fade to prevent harsh artifacts at
the transition line
reworked sky rendering to accept fog by using the surface renderer
reworked sprite rendering to use the surface renderer
reworked corona rendering to use the surface renderer
reworked fogging a bit on nomodel entities
eliminated R_DrawSprite

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@9437 d7cf8633-e32d-0410-b094-e92efae38249