-Rewrite sprite projection and clipping. Sprites are clipped against the lines x = y*fovtan and -x = y*fovtan similar to linesegs in R_AddLine.
-Added clipping of sprites that are entirely above or below the viewing window.

git-svn-id: http://odamex.net/svn/root/branches/persdiv@3660 b0bcef46-2027-0410-bd56-ab726e9a1d04