Separate Fl_GIF_Image constructors with and w/o length arg

Document clearly that reading from memory w/o the length argument
(old constructor) is discouraged (deprecated).

FL/Fl_GIF_Image.H

@@ -31,7 +31,10 @@ 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, const long length = -1);
+  // deprecated constructor w/o length (for backwards compatibility)
+  Fl_GIF_Image(const char* imagename, const unsigned char *data);
+  // constructor with length (since 1.4.0)
+  Fl_GIF_Image(const char* imagename, const unsigned char *data, const size_t length);
 
 protected:
 

src/Fl_GIF_Image.cxx

@@ -146,16 +146,8 @@ Fl_GIF_Image::Fl_GIF_Image(const char *filename) :
   The destructor frees all memory and server resources that are used by
   the image.
 
-  The (new and optional) third parameter \p length \b should be used so buffer
-  overruns (i.e. truncated images) can be checked. See note below.
-
-  If \p length is not used
-  - it defaults to -1 (unlimited size)
-  - buffer overruns will not be checked.
-
-  \note The optional parameter \p length is available since FLTK 1.4.0.
-    Not using it is deprecated and old code should be modified to use it.
-    This parameter will likely become mandatory in a future FLTK version.
+  The third parameter \p length is used to test for buffer overruns,
+  i.e. truncated images.
 
   Use Fl_Image::fail() to check if Fl_GIF_Image failed to load. fail() returns
   ERR_FILE_ACCESS if the file could not be opened or read, ERR_FORMAT if the
@@ -168,8 +160,10 @@ Fl_GIF_Image::Fl_GIF_Image(const char *filename) :
 
   \see Fl_GIF_Image::Fl_GIF_Image(const char *filename)
   \see Fl_Shared_Image
+
+  \since 1.4.0
 */
-Fl_GIF_Image::Fl_GIF_Image(const char *imagename, const unsigned char *data, const long length) :
+Fl_GIF_Image::Fl_GIF_Image(const char *imagename, const unsigned char *data, const size_t length) :
   Fl_Pixmap((char *const*)0)
 {
   Fl_Image_Reader rdr;
@@ -180,10 +174,43 @@ Fl_GIF_Image::Fl_GIF_Image(const char *imagename, const unsigned char *data, con
   }
 }
 
+/**
+  This constructor loads a GIF image from memory (deprecated).
+
+  \deprecated Please use
+    Fl_GIF_Image(const char *imagename, const unsigned char *data, const size_t length)
+    instead.
+
+  \note Buffer overruns will not be checked.
+
+  This constructor should not be used because the caller can't supply the
+  memory size and the image reader can't check for "end of memory" errors.
+
+  \note A new constructor with parameter \p length is available since FLTK 1.4.0.
+
+  \param[in] imagename  A name given to this image or NULL
+  \param[in] data       Pointer to the start of the GIF image in memory.
+
+  \see Fl_GIF_Image(const char *filename)
+  \see Fl_GIF_Image(const char *imagename, const unsigned char *data, const size_t length)
+*/
+Fl_GIF_Image::Fl_GIF_Image(const char *imagename, const unsigned char *data) :
+  Fl_Pixmap((char *const*)0)
+{
+  Fl_Image_Reader rdr;
+  if (rdr.open(imagename, data) == -1) {
+    ld(ERR_FILE_ACCESS);
+  } else {
+    load_gif_(rdr);
+  }
+}
+
 /*
- This method reads GIF image data and creates an RGB or RGBA image. The GIF
- format supports only 1 bit for alpha. To avoid code duplication, we use
- an Fl_Image_Reader that reads data from either a file or from memory.
+  This method reads GIF image data and creates an RGB or RGBA image. The GIF
+  format supports only 1 bit for alpha. The final image data is stored in
+  a modified XPM format (Fl_GIF_Image is a subclass of Fl_Pixmap).
+  To avoid code duplication, we use an Fl_Image_Reader that reads data from
+  either a file or from memory.
 */
 void Fl_GIF_Image::load_gif_(Fl_Image_Reader &rdr)
 {