Fix display of R2L lines when the last character fits only partially.

Fix display of R2L lines when the last character fits only partially.

 See http://lists.gnu.org/archive/html/emacs-devel/2014-07/msg00476.html
 for the details.
 src/xdisp.c (extend_face_to_end_of_line): If the last glyph of an
 R2L row is visible only partially, give the row a negative x
 offset.
 (display_line): Fix the calculation of the glyph whose pixel width
 is used to decide whether the last produced glyph fits on the
 line.  When the last glyph fits only partially, give the row a
 negative x offset.