Allow modal progress sleep to finish early on completion

src/progress.h

@@ -39,6 +39,7 @@ static inline bool UseThreadedModelProgress()
 
 bool IsFirstModalProgressLoop();
 void SetModalProgress(bool state);
+void SleepWhileModalProgress(int milliseconds);
 
 extern std::mutex _modal_progress_work_mutex;
 extern std::mutex _modal_progress_paint_mutex;