Add centiseconds to timer overlays

This adds centiseconds to the in-game timer, as well as the time trial
timer.

This is to aid speedrun moderators in determining when exactly a run was
completed, which they can't easily do if the timer only has a precision
up to a second.

desktop_version/src/Game.h

@@ -143,6 +143,8 @@ public:
 
     std::string timetstring(int t);
 
+    void timestringcenti(char* buffer, size_t buffer_size);
+
     void returnmenu(void);
     void returntomenu(enum Menu::MenuName t);
     void  createmenu(enum Menu::MenuName t, bool samemenu = false);