Improve fix for bug #11832 with truncated/continued lines w/o margins.

Improve fix for bug #11832 with truncated/continued lines w/o margins.

 src/xdisp.c (insert_left_trunc_glyphs): Fix incorrect size in
 memmove call that removes glyphs covered by the left truncation
 glyph.  Improve commentary.
 (display_line): Fix display of continuation glyphs on GUI frames
 when the right fringe is turned off and variable-size fonts are
 used in the window.  Move the code that appends a stretch glyph to
 produce_special_glyphs, so that it could be used for truncation
 and continuation glyphs alike.
 (produce_special_glyphs) [HAVE_WINDOW_SYSTEM]: Produce a stretch
 glyph of a suitably computed width, to align the special glyphs at
 the window margin.  Code moved from display_line.