(Voverlay_arrow_variable_list): New variable to properly

(Voverlay_arrow_variable_list): New variable to properly
implement and integrate multiple overlay arrows with redisplay.
(syms_of_xdisp): DEFVAR_LISP and initialize it.
(last_arrow_position, last_arrow_string): Replace by properties.
(Qlast_arrow_position, Qlast_arrow_string)
(Qoverlay_arrow_string, Qoverlay_arrow_bitmap): New variables.
(syms_of_xdisp): Intern and staticpro them.
(overlay_arrow_string_or_property, update_overlay_arrows)
(overlay_arrow_in_current_buffer_p, overlay_arrows_changed_p)
(overlay_arrow_at_row): New functions for multiple overlay arrows.
(redisplay_internal): Use them instead of directly accessing
Voverlay_arrow_position etc. for multiple overlay arrows.
(mark_window_display_accurate): Use update_overlay_arrows.
(try_cursor_movement): Use overlay_arrow_in_current_buffer_p.
(try_window_id): Use overlay_arrows_changed_p.
(get_overlay_arrow_glyph_row): Add overlay_arrow_string arg.
(display_line): Use overlay_arrow_at_row to check multiple
overlay arrows, and get relevant overlay-arrow-string and
overlay-arrow-bitmap.  Set w->overlay_arrow_bitmap accordingly.
(produce_image_glyph): Set pixel_width = 0 for fringe bitmap.
(syms_of_xdisp): Remove last_arrow_position and last_arrow_string.