Fix display of R2L lines and cursor motion in bidi buffers.

Fix display of R2L lines and cursor motion in bidi buffers.

 xdisp.c (find_row_end): New function, refactored from display_line.
 (display_line): Use it.
 (extend_face_to_end_of_line): In almost-filled rows, extend only
 if the row is R2L and not continued.
 (display_line): Fix prepending of truncation glyphs to R2L rows.
 Preserve overlay and string info in row->end.
 (insert_left_trunc_glyphs): Support addition of left truncation
 glyphs to R2L rows.
 (set_cursor_from_row): Don't place cursor on the vertical border
 glyph between adjacent windows.  Fix a crash when a display string
 is continued to the next line.  Don't return zero if cursor was
 found by `cursor' property of a display string.
 (try_cursor_movement): Don't assume that row->end == (row+1)->start,
 test for that explicitly.