Fix timedemo in OpenGL mode

Fix fps display in timedemo in OpenGL mode.

Before:

Timing demo 'replay/online/1598123606-dark-race-srb2kart-server.lmp'.
Playing back demo 'replay/online/1598123606-dark-race-srb2kart-server.lmp'.
Pardon me while I initialize the Lua scripting interface...
timed 2089 gametics in 133 realtics
3.800000 seconds, 0.000000 avg fps```

After:

```$timedemo replay/online/1598123606-dark-race-srb2kart-server.lmp
Timing demo 'replay/online/1598123606-dark-race-srb2kart-server.lmp'.
Playing back demo 'replay/online/1598123606-dark-race-srb2kart-server.lmp'.
Pardon me while I initialize the Lua scripting interface...
timed 2089 gametics in 125 realtics
3.571429 seconds, 584.640000 avg fps```