Fix a fluid crash when writing a template screenshot

We need to show() the window before we call fl_read_image() if it
is not shown yet, otherwise fluid would crash.

fluid/Fl_Window_Type.cxx

@@ -425,8 +425,14 @@ void Fl_Window_Type::open() {
 uchar *Fl_Window_Type::read_image(int &ww, int &hh) {
   Overlay_Window *w = (Overlay_Window *)o;
 
+  int hidden = !w->shown();
+  w->show(); // make it the front window
+
   // Read the screen image...
-  return (w->read_image(ww, hh));
+  uchar *idata = w->read_image(ww, hh);
+  if (hidden)
+    w->hide();
+  return idata;
 }