Char-table defining glyphless characters.
Each element, if non-nil, should be one of the following:
an ASCII acronym string: display this string in a box
`hex-code': display the hexadecimal code of a character in a box
`empty-box': display as an empty box
`thin-space': display as 1-pixel width space
`zero-width': don't display
An element may also be a cons cell (GRAPHICAL . TEXT), which specifies the
display method for graphical terminals and text terminals respectively.
GRAPHICAL and TEXT should each have one of the values listed above.
The char-table has one extra slot to control the display of a character for
which no font is found. This slot only takes effect on graphical terminals.
Its value should be an ASCII acronym string, `hex-code', `empty-box', or `thin-space'. The default is `empty-box'.