Rename generic x_* procedures in xdisp.c

Rename generic x_* procedures in xdisp.c

* src/xdisp.c (x_consider_frame_title, x_get_glyph_overhangs)
(x_produce_glyphs, x_write_glyphs, x_insert_glyphs)
(x_clear_end_of_line), x_fix_overlapping_area)
(x_update_cursor, x_clear_cursor, x_clear_window_mouse_face)
(x_draw_vertical_border, x_draw_right_divider, x_draw_bottom_divider)
(x_intersect_rectangles): Rename with a gui prefix to indicate
non-X-specific functionality.

* src/composite.c:
* src/dispextern.h:
* src/nsfns.m:
* src/nsterm.m:
* src/w32fns.c:
* src/w32term.c:
* src/window.c:
* src/xfns.c:
* src/xterm.c: Use the renamed procedures.