Given a graphical location in the buffer, returns the item buffer position at that location.
Only valid when the buffer is viewed.
See section
for a discussion of the at-eol? argument.
The on-it? box is filled with #t if the line actually touches this position, or #f otherwise, unless on-it? is null.
The edge-close? box is filled with it will be filled in with a value indicating how close the point is to the vertical edges of the item when the point falls on the item, unless edge-close? is null. If the point is closest to the left edge of the item, the value will be negative; otherwise, the value will be positive. In either case, then absolute value of the returned result is the distance from the point to the edge of the item. The values 100 and -100 indicate infinity.