Fix Fl::add_timeout() in draw() under Linux (STR 3188)

Timeouts queued during Fl_Widget::draw() - called by Fl::flush() -
would not be serviced unless other events were processed. For
Details see STR 3188.

CHANGES.txt

@@ -105,6 +105,7 @@ Changes in FLTK 1.4.0					Released: ??? ?? 2019
   Other Improvements
 
   - (add new items here)
+  - Fix Fl::add_timeout() in draw() under Linux (STR 3188)
   - Class Fl_Native_File_Chooser uses in priority the file chooser dialog
     of GTK version 3 when available on the running X11 platform, and falls back
     to version 2 when V3 is not available. In contrast, GTK version 2 was used

src/drivers/X11/Fl_X11_Screen_Driver.cxx

@@ -480,6 +480,10 @@ double Fl_X11_Screen_Driver::wait(double time_to_wait)
     Fl::flush();
     if (Fl::idle && !in_idle) // 'idle' may have been set within flush()
       time_to_wait = 0.0;
+    else if (first_timeout && first_timeout->time < time_to_wait) {
+      // another timeout may have been queued within flush(), see STR #3188
+      time_to_wait = first_timeout->time >= 0.0 ? first_timeout->time : 0.0;
+    }
     return this->poll_or_select_with_delay(time_to_wait);
   }
 }