Make Fl_Image_Reader::name() 'const' (#271)

Thanks to @wcout for finding this.

src/Fl_Image_Reader.h

@@ -80,7 +80,7 @@ public:
   }
 
   // return the name or filename for this reader
-  const char *name() { return pName; }
+  const char *name() const { return pName; }
 
   // skip a given number of bytes
   void skip(unsigned int n) { seek(tell() + n); }