Merge towerlogic() into gamelogic()

This doesn't change anything functionality-wise, but it does remove a
lot of duplicate code, which makes it easier to work on.

desktop_version/src/main.cpp

@@ -426,7 +426,7 @@ int main(int argc, char *argv[])
                 {
                     gameinput();
                     towerrender();
-                    towerlogic();
+                    gamelogic();
 
                 }
                 else