Curl_pgrsStartNow: clear all bits except HIDE

Bug: http://curl.haxx.se/bug/view.cgi?id=3385258
Reported by: Ben Winslow

lib/progress.c

@@ -201,7 +201,7 @@ void Curl_pgrsStartNow(struct SessionHandle *data)
 {
   data->progress.speeder_c = 0; /* reset the progress meter display */
   data->progress.start = Curl_tvnow();
-  data->progress.flags = 0;
+  data->progress.flags &= ~PGRS_HIDE; /* clear all bits except HIDE */
 }
 
 void Curl_pgrsSetDownloadCounter(struct SessionHandle *data, curl_off_t size)