Merge towerrender() into gamerender()

This removes a lot of duplicate code, which towerrender() mostly
consisted of, even though the only difference is that it draws a
separate map and screen edge spikes are drawn.

desktop_version/src/main.cpp

@@ -425,7 +425,7 @@ int main(int argc, char *argv[])
                 if (map.towermode)
                 {
                     gameinput();
-                    towerrender();
+                    gamerender();
                     gamelogic();
 
                 }