Converts from X and Y position within a layout to the byte index to the character at that logical position.
If the Y position is not inside the layout, the closest position is chosen (the position will be clamped inside the layout). If the X position
is not within the layout, then the start or the end of the line is chosen as described for
x_to_index. If either the X or Y positions were not inside the layout, then
the function returns
false; on an exact hit, it returns
the X offset (in Pango units) from the left edge of the layout.
the Y offset (in Pango units) from the top edge of the layout
location to store calculated byte index
location to store a integer indicating where in the grapheme the user clicked. It will either be zero, or the number of characters in the grapheme. 0 represents the leading edge of the grapheme.