Truncate string STR to end at column END-COLUMN.
The optional 3rd arg START-COLUMN, if non-nil, specifies the starting
column; that means to return the characters occupying columns
START-COLUMN ... END-COLUMN of STR. Both END-COLUMN and START-COLUMN
are specified in terms of character display width in the current
buffer; see also `char-width'.
The optional 4th arg PADDING, if non-nil, specifies a padding
character (which should have a display width of 1) to add at the end
of the result if STR doesn't reach column END-COLUMN, or if END-COLUMN
comes in the middle of a character in STR. PADDING is also added at
the beginning of the result if column START-COLUMN appears in the
middle of a character in STR.
If PADDING is nil, no padding is added in these cases, so
the resulting string may be narrower than END-COLUMN.
If ELLIPSIS is non-nil, it should be a string which will replace the
end of STR (including any padding) if it extends beyond END-COLUMN,
unless the display width of STR is equal to or less than the display
width of ELLIPSIS. If it is non-nil and not a string, then ELLIPSIS
defaults to "...".