Add a fast-forward keybind to level debugger

This lets you hold down F to fast-forward the game if you have the level
debugger interface open (with Y) and the game isn't paused.

This is most useful for quickly skipping through cutscenes to test
something.

desktop_version/src/Game.cpp

@@ -7503,6 +7503,14 @@ static inline int get_framerate(const int slowdown)
 
 int Game::get_timestep(void)
 {
+    if ((gamestate == GAMEMODE || (gamestate == TELEPORTERMODE && !useteleporter)) &&
+    level_debugger::is_active() &&
+    !level_debugger::is_pausing() &&
+    key.isDown(SDLK_f))
+    {
+        return 1;
+    }
+
     switch (gamestate)
     {
     case GAMEMODE: