the text for the run
the number of bytes (not characters) in @text.
the analysis information return from [func@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.
The X position is measured from the left edge of the run. Character positions are computed by dividing up each cluster into equal portions.