the text for the run
the number of bytes (not characters) in text.
the analysis information return from pango_itemize()
the byte index within text
whether we should compute the result for the beginning (FALSE) or end (TRUE) of the character.
location to store result
Converts from character position to x position. (X position is measured from the left edge of the run). Character positions are computed by dividing up each cluster into equal portions.