Made the renderer modular and loadable

This removes coupling between the renderer and UI/client functions. An option USE_RENDERER_DLOPEN was added to specify whether a renderer module should be compiled and loaded, instead of integrating the renderer into the executable directly. This opens the door for a new renderer

code/renderergl1/tr_vis.cpp

@@ -335,8 +335,8 @@ void R_VisDebug()
         LoadPortals(buffer);
     }
 
-    leafnum = CM_PointLeafnum(tr.refdef.vieworg);
-    cluster = CM_LeafCluster(leafnum);
+    leafnum = ri.CM_PointLeafnum(tr.refdef.vieworg);
+    cluster = ri.CM_LeafCluster(leafnum);
     if (cluster < 0) {
         return;
     }