Fix display of stretches of whitespace in the display margins

Fix display of stretches of whitespace in the display margins

* src/xdisp.c (produce_stretch_glyph): Truncate the stretch glyph
due to line wrap only when drawing in the text area.
* src/xterm.c (x_draw_stretch_glyph_string):
* src/w32term.c (w32_draw_stretch_glyph_string): Fix the
adjustment of the stretch X and width so that stretch glyphs could
be drawn in the left margin.  Reported by Paul W. Rankin
<pwr@bydasein.com>.