Interpolate spikeleveltop and spikelevelbottom

This doesn't have much effect, except for when the spikes quickly move,
because the spikes only usually move at 1 pixel per frame anyway.

desktop_version/src/Logic.cpp

@@ -261,6 +261,8 @@ void gamelogic()
     if (map.towermode)
     {
         map.oldypos = map.ypos;
+        map.oldspikeleveltop = map.spikeleveltop;
+        map.oldspikelevelbottom = map.spikelevelbottom;
         if(!game.completestop)
         {
             if (map.cameramode == 0)