Sprite sheet generation can go out of bounds

[pushfoo]

TL;DR: Font indices go OOB when writing fonts

Reported on Windows (ty @DragonMoffon) and confirmed on Linux.

Replication steps:

1. Install main branch from source in a fresh venv
2. Go to https://int10h.org/oldschool-pc-fonts/download/
3. Download the latest font pack
4. Navigate into the ttf - Mx (mixed outline+bitmap) folder of the zip
5. Extract MxPlus_IBM_BIOS.ttf
6. fontknife convert -g " ! ~\$%&'()+*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_\'abcdefghijklmnopqrstuvwxyz{|}~ ÇüéâäàåçêëèïîìÄÅÉæÆôöòûùÿÖÜ¢£¥₧ƒáíóúñѪº¿⌐¬½¼¡«»░▒▓│┤╡╢╖╕╣║╗╝╜╛┐└┴┬├─┼╞╟╚╔╩╦╠═╬╧╨╤╥╙╘╒╓╫╪┘┌█▄▌▐▀αßΓπΣσµτΦΘΩδ∞φε∩≡±≥≤⌠⌡÷≈°∙·√ⁿ²■ " /home/user/Downloads/MxPlus_IBM_BIOS.ttf test.png
7. Observe issue below:

$ fontknife convert -g " ! ~\$%&'()+*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_\'abcdefghijklmnopqrstuvwxyz{|}~ ÇüéâäàåçêëèïîìÄÅÉæÆôöòûùÿÖÜ¢£¥₧ƒáíóúñѪº¿⌐¬½¼¡«»░▒▓│┤╡╢╖╕╣║╗╝╜╛┐└┴┬├─┼╞╟╚╔╩╦╠═╬╧╨╤╥╙╘╒╓╫╪┘┌█▄▌▐▀αßΓπΣσµτΦΘΩδ∞φε∩≡±≥≤⌠⌡÷≈°∙·√ⁿ²■ " /home/user/Downloads/MxPlus_IBM_BIOS.ttf test.png
Traceback (most recent call last):
  File "/home/user/Projects/Fontknife/.venv/bin/fontknife", line 8, in <module>
    sys.exit(main())
  File "/home/user/Projects/Fontknife/fontknife/frontend/__main__.py", line 117, in main
    args.callback(font, output_path, output_kwargs)
  File "/home/user/Projects/Fontknife/fontknife/frontend/commands/convert.py", line 20, in main
    write_font(font, output, **output_args)
  File "/home/user/Projects/Fontknife/fontknife/formats/__init__.py", line 105, in write_font
    writer.write_output(font, output, **kwargs)
  File "/home/user/Projects/Fontknife/fontknife/formats/writers/spritesheet.py", line 62, in write_output
    paste_bbox = grid_mapper.bbox_for_sheet_index(index)
  File "/home/user/Projects/Fontknife/fontknife/formats/common/spritesheet.py", line 193, in bbox_for_sheet_index
    coord = self.coord_for_sheet_index(index)
  File "/home/user/Projects/Fontknife/fontknife/formats/common/spritesheet.py", line 185, in coord_for_sheet_index
    raise e
KeyError: 'Index 96 not in grid'