textview: fix assertion when toggling caret visibility

When toggling caret visibility (with F7) we would hit an assertion if the
cursor is currently blinking. This adjusts things to ensure that we should
be showing the carent when scheduling our blink timeouts.

Fixes #2647

gtk/gtktextview.c

@@ -6048,7 +6048,7 @@ gtk_text_view_check_cursor_blink (GtkTextView *text_view)
 {
   GtkTextViewPrivate *priv = text_view->priv;
 
-  if (cursor_blinks (text_view))
+  if (cursor_blinks (text_view) && cursor_visible (text_view))
     {
       if (!priv->blink_tick)
         add_blink_timeout (text_view, FALSE);