Consistently use marker_position and marker_byte_position.

Consistently use marker_position and marker_byte_position.
* fringe.c (Ffringe_bitmaps_at_pos):
* indent.c (Fvertical_motion):
* insdel.c (prepare_to_modify_buffer):
* keyboard.c (make_lispy_position):
* window.c (Fwindow_end, Fpos_visible_in_window_p, unshow_buffer)
(window_scroll_pixel_based, displayed_window_lines)
(Fset_window_configuration):
* xdisp.c (message_dolog, with_echo_area_buffer_unwind_data)
(mark_window_display_accurate_1, redisplay_window, decode_mode_spec):
Replace direct access to marker fields with calls
to marker_position and/or marker_byte_position.