public bool x_to_index (int x_pos, out int index_, out int trailing)
Converts from x offset to the byte index of the corresponding character within the text of the layout.
If x_pos is outside the line, index_ and trailing will point to the very first or very last
position in the line. This determination is based on the resolved direction of the paragraph; for example, if the resolved direction
is right-to-left, then an X position to the right of the line (after it) results in 0 being stored in index_ and
trailing. An X position to the left of the line results in index_ pointing to the (logical) last grapheme in the
line and trailing being set to the number of characters in that grapheme. The reverse is true for a left-to-right line.