convert_background should ignore filler byte for game_over.tilemap

[kemenaran]

$ backgrounds % ../../../../../tools/convert_background.py decode game_over.tilemap.encoded --filler 0x7e --output game_over.tilemap
$ backgrounds % ../../../../../tools/convert_background.py encode game_over.tilemap --filler 0x7e --output game_over.tilemap.encoded

From @KelseyHigham, the filler byte is not ignored in the encoded output (but it should be).