Fix ghost buffer boundary indicators in fringes (Bug#5634, Bug#6325).

Fix ghost buffer boundary indicators in fringes (Bug#5634, Bug#6325).

* dispextern.h (FRINGE_HEIGHT_BITS): New define.
(struct glyph_row): New members left_fringe_offset and
right_fringe_offset.

* xterm.c (x_draw_fringe_bitmap): Don't clip bottom aligned bitmap
specially.
* w32term.c (w32_draw_fringe_bitmap): Likewise.
* nsterm.m (ns_draw_fringe_bitmap): Likewise.

* fringe.c (draw_fringe_bitmap_1): Don't clip bitmap here.
Take account of	bitmap offset.
(draw_window_fringes): Take account of window vscroll.
(update_window_fringes): Likewise.  Extend top-aligned top indicator
or bottom-aligned bottom indicator to adjacent rows if it doesn't fit
in one row.  Don't set redraw_fringe_bitmaps_p outside row comparison.
Set left_fringe_offset and right_fringe_offset (Bug#5634, Bug#6325).