CalcSurfaceExtents now uses surf->poly_verts instead of edges

CalcSurfaceExtents now uses surf->poly_verts instead of edges
brush model bounding box calculations are now based on meshes instead of edges
CalcSurfaceExtents has been merged into Mod_GenerateSurfacePolygon
msurface_t firstedge and numedges have been removed (they were only used for generating the polygon during loading)
shuffled around msurface_t fields in order of most common access at the top, to least common at the bottom, this is in an attempt to reduce wasted memory traffic

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@2640 d7cf8633-e32d-0410-b094-e92efae38249