added rmesh_t and R_Mesh_AddVertex3f, R_Mesh_AddPolygon3f, R_Mesh_AddBrushMeshFromPla...

added rmesh_t and R_Mesh_AddVertex3f, R_Mesh_AddPolygon3f, R_Mesh_AddBrushMeshFromPlanes functions
R_Shadow_ScissorForBBox now uses R_Mesh_AddBrushMeshFromPlanes function to get an exact clipped box-brush vertex set for calculating scissor area, this should reduce the scissor area used in some cases compared to the old hacky method
added nearclip plane to frustum[] array (only used by R_Shadow_ScissorForBBox)

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@5247 d7cf8633-e32d-0410-b094-e92efae38249