(init_from_display_pos): If POS is in an overlay string,

(init_from_display_pos): If POS is in an overlay string,
deal with the first overlay string having an image `display'
property.
(try_window_reusing_current_matrix, compute_line_metrics): Fix
computation of row's visible height for the case that part of the
row is invisible above and part of the row is at the same time
invisible below the window.