Fix GIF save - last byte

The last byte was not saved for sure, depending on the codesize.
Also if the codesize was big (12 bits which is the max)
in some case an additional 0 byte was store (which is harmless but
useless)

This bug only appears if the bit per code are < 8
which is possible since commit 834050c2c4

src/fileformats.c

@@ -3017,7 +3017,12 @@ void Save_GIF(T_IO_Context * context)
     
                   GIF_set_code(eof);  // 257 for 8bpp    // Code de End d'image
                   if (GIF_remainder_bits!=0)
-                    GIF_set_code(0);             // Code bidon permettant de s'assurer que tous les bits du dernier code aient bien étés inscris dans le buffer GIF
+                  {
+                    // WRITE last byte
+                    GIF_buffer[++GIF_remainder_byte]=GIF_last_byte;
+                    GIF_last_byte=0;
+                    GIF_remainder_bits=0;
+                  }
                   GIF_empty_buffer();         // On envoie les dernières données du buffer GIF dans le buffer KM
     
                   // On écrit un \0