(zv_height, zv_bits, zv_period): Changed zv bitmap to

(zv_height, zv_bits, zv_period): Changed zv bitmap to
fill fringe evenly with small dashes.
(x_draw_fringe_bitmap): Clear background if necessary. Align and
clip the new ZV bitmap to avoid jitter between rows.
(x_draw_row_fringe_bitmaps): Rely on x_draw_fringe_bitmap to clear
background.  Don't draw fringe bitmaps if fringe width is zero.
(x_compute_fringe_widths): New function.
(x_new_font, x_set_window_size_1): Use it.