Why GtkTextIter doesn't provide a function to return its byte index? How to retrieve it?

[Gang Chen <gang chen cn gmail com>]

Hi all,

My application has its own text buffer which uses byte indexes to
index the text. I create a GtkTextBuffer as a mirror of the text
buffer in order to display the text in a GtkTextView. I need to
retrieve the byte index of a GtkTextIter but found no such function
exists. How to?

Thanks,
Gang