public void letter_space (string text, LogAttr log_attrs, int letter_spacing)
Adds spacing between the graphemes of this to give the effect of typographic letter spacing.
text that this corresponds to (glyph_item->item->offset is an offset from the start of
logical attributes for the item (the first logical attribute refers to the position before the first character in the item)
amount of letter spacing to add in Pango units. May be negative, though too large negative values will give ugly results.