Converts a (x,y) coordinate in an item position.
int IupConvertXYToPos(Ihandle *ih, int x, int y); [in C] iup.ConvertXYToPos(ih: ihandle, x, y: number) -> (ret: number) [in Lua]
ih: Identifier of the element.
x: X coordinate relative to the left corner of the element.
y: Y coordinate relative to the top corner of the element.
Returns: the position starting at 0 (except for IupList that starts at 1). If fails returns -1.
It can be used for IupText and IupScintilla (returns a position in the string), IupList (returns an item), IupTree (returns a node identifier) or IupMatrix (returns a cell position, where pos=lin*numcol + col).