Add two virtual methods to class Fl_Image

(1) The new virtual method Fl_Image::release() which is equivalent to
  'delete this' automatically extends to Fl_Shared_Image::release()
  which makes the latter method virtual.

  This new method in the base class makes Fl_Image::release() callable
  on all objects derived from Fl_Image.

(2) Add virtual method Fl_Shared_Image *Fl_Image::as_shared_image()

  This new method can be used to detect whether an Fl_Image instance
  is an Fl_Shared_Image or not.