Increase speed of rewind

The commit increases very much speed of demo rewind and also prevents
AC freeze for gamespeed = 1000.
It may degrade physics for gamespeed = 1000 on very low end machines.

source/src/physics.cpp

@@ -755,6 +755,7 @@ void physicsframe()          // optimally schedule physics frames inside the gra
         physframetime = clamp((PHYSFRAMETIME*gamespeed)/100, 1, PHYSFRAMETIME);
         physsteps = (diff + physframetime - 1)/physframetime;
         lastphysframe += physsteps * physframetime;
+        if(!multiplayer(false) && physsteps > 1000) physsteps = 1000;
     }
 }