Fix bug #11832 with truncated lines when fringes are disabled on GUI frames.

Fix bug #11832 with truncated lines when fringes are disabled on GUI frames.

 src/xdisp.c (init_iterator): Get dimensions of truncation and
 continuation glyphs even if on GUI frames.  Adjust
 it->last_visible_x on GUI frames when the left or right fringes,
 or both, are absent.
 (start_display, move_it_in_display_line_to): Handle the case of a
 GUI frame without a fringe to display continuation or truncation
 glyphs.
 (insert_left_trunc_glyphs): Support GUI frames: make sure
 truncation glyphs overwrite enough glyphs from the current line to
 have sufficient space in pixels.
 (display_line): Support truncation and continuation glyphs on GUI
 frames.  If some spare pixels are left on the line after inserting
 the truncation glyphs, fill that space with a stretch glyph of a
 suitably computed width.
 src/term.c (produce_special_glyphs): Call PRODUCE_GLYPHS, not
 produce_glyphs, to support GUI sessions.