Re-apply c0616750

Lost in merge: e95216b5

src/gfx.cpp

@@ -1551,7 +1551,7 @@ void DrawDirtyBlocks()
 		_modal_progress_work_mutex.unlock();
 
 		/* Wait a while and hope the modal gives us a bit of time to draw the GUI. */
-		if (!is_first_modal_progress_loop) CSleep(MODAL_PROGRESS_REDRAW_TIMEOUT);
+		if (!is_first_modal_progress_loop) SleepWhileModalProgress(MODAL_PROGRESS_REDRAW_TIMEOUT);
 
 		/* Modal progress thread may need blitter access while we are waiting for it. */
 		VideoDriver::GetInstance()->ReleaseBlitterLock();