Fix and improve Fl_GIF_Image (Issue fltk#271, fltk#274)

- add error and EOF checks
- fix transparent pixel index outside ColorMap (Issue fltk#271)
- fix Fl_GIF_Image decoder bug (Issue fltk#274)

Work in progress, there's still room for improvements.

FL/Fl_GIF_Image.H

@@ -1,7 +1,7 @@
 //
 // GIF image header file for the Fast Light Tool Kit (FLTK).
 //
-// Copyright 1998-2020 by Bill Spitzak and others.
+// Copyright 1998-2021 by Bill Spitzak and others.
 //
 // This library is free software. Distribution and use rights are outlined in
 // the file "COPYING" which should have been included with this file.  If this
@@ -31,7 +31,7 @@ class FL_EXPORT Fl_GIF_Image : public Fl_Pixmap {
 public:
 
   Fl_GIF_Image(const char* filename);
-  Fl_GIF_Image(const char* imagename, const unsigned char *data);
+  Fl_GIF_Image(const char* imagename, const unsigned char *data, const long length = -1);
 
 protected: