add support for reading HP-48 Grob format

src/const.h

@@ -155,6 +155,7 @@ enum FILE_FORMATS
   FORMAT_MOTO, ///< Thomson MO/TO computers pictures
   FORMAT_HGR,  ///< Apple II HGR and DHGR
   FORMAT_TIFF, ///< Tagged Image File Format
+  FORMAT_GRB,  ///< HP-48 Grob
   FORMAT_MISC, ///< Must be last of enum: others formats recognized by SDL_image (or recoil)
   FORMAT_CLIPBOARD  ///< To load/save from/to Clipboard
 };