R_CullSurfaces: small optimization. Avoid doing (s->visframe == r_visframecount) test on all world surfaces, instead, use the surfaces in the world texture chains, which already passed that test in R_MarkSurfaces.

git-svn-id: svn://svn.code.sf.net/p/quakespasm/code/trunk/quakespasm@991 af15c1b1-3010-417e-b628-4374ebc0bcbd