Added ansi_show_unknown(bool) (default off)

It may be useful to some to have the terminal emit an error character
to show unknown escape sequences. Off by default, unknown escape sequences
are silently ignored. If enabled, '¿' is inserted instead.

FL/Fl_Simple_Terminal.H

@@ -161,7 +161,6 @@ private:
 private:
   int  history_lines_;      // max lines allowed in screen history
   bool stay_at_bottom_;     // lets scroller chase last line in buffer
-  bool ansi_;               // enables ANSI sequences
   // scroll management
   int  lines_;              // #lines in buffer (optimization: Fl_Text_Buffer slow to calc this)
   bool scrollaway_;         // true when user changed vscroll away from bottom
@@ -175,7 +174,10 @@ private:
   int  normal_style_index_;  // "normal" style used by "\033[0m" reset sequence
   int  current_style_index_; // current style used for drawing text
   char current_style_;       // current 'style char' (e.g. 'A' = first style entry)
+  // ANSI escape seq
   Fl_Escape_Seq escseq;      // escape sequence state handler
+  bool ansi_;                // enables ANSI sequences
+  bool ansi_show_unknown_;   // show '¿' for unknown ESC sequences (default: off)
   // String parsing vars initialized/used by append(), used by handle_backspace() etc.
   char *ntm_;                // new text memory  (ntm) - malloc()ed by append() for output text
   char *ntp_;                // new text ptr     (ntp) - points into ntm buffer
@@ -193,6 +195,8 @@ public:
   int  history_lines() const;
   void ansi(bool val);
   bool ansi() const;
+  void ansi_show_unknown(bool val);
+  bool ansi_show_unknown() const;
   void style_table(Fl_Text_Display::Style_Table_Entry *stable, int stable_size, int normal_style_index=0);
   const Fl_Text_Display::Style_Table_Entry *style_table() const;
   int style_table_size() const;
@@ -234,6 +238,7 @@ protected:
   void backspace_buffer(unsigned int count);
   void handle_backspace();
   void append_ansi(const char *s, int len);
+  void unknown_escape();
 };
 
 #endif