Minor documentation fix.
[autofit] Fix Savannah bug #42148.

The adaptation of the cjk auto-hinter module to blue stringsets in
2013-08-25 had three severe bugs.  Mea culpa.

1. Contrary to the latin auto-hinter, characters for reference and
   overshoot values of a blue zone are specified separately.  Due to
   the screwed-up change it didn't work at all.

2. A boolean comparison was erroneously replaced with a cast,
   causing invalid results with the `^' operator later on.  The
   visual artifact caused by this problem is the topic of the bug
   report.

3. Two flag values were inverted, causing incorrect assignment of
   reference and overshoot values.

* src/autofit/afblue.dat: Fix CJK bluestrings, introducing a new
syntax to have both reference and overshoot characters in a single
string.  This is error #1.
Add extensive comments.

* src/autofit/afblue.hin (AF_BLUE_PROPERTY_CJK_FILL): Removed, no
longer used.
(AF_BLUE_PROPERTY_CJK_TOP, AF_BLUE_PROPERTY_CJK_HORIZ): Fix values.
This is error #3.

* src/autofit/afblue.c, src/autofit/afblue.h: Regenerated.

* src/autofit/afcjk.c (af_cjk_metrics_init_blues): Correct error #1.
Use character `|' to separate characters for reference and overshoot
values.
Improve tracing messages, synchronizing them with the latin
auto-hinter.
(af_cjk_hints_compute_blue_edges): Fix value of `is_top_right_blue'.
This is error #2.
(af_cjk_align_linked_edge): Add tracing message.

* src/autofit/afcjk.h (AF_CJK_IS_FILLED_BLUE): Removed, no longer
used.
* src/sfnt/pngshim.c (error_callback): s/longjmp/ft_longjmp/.
* src/raster/ftraster.c (Vertical_Sweep_Span): Use drop-out mode.

This spot has been missed while introducing support for various
drop-out modes years ago (including no drop-out mode, which this
commit fixes).

Problem reported by Patrick Thomas <pthomas505@gmail.com>.
Minor.
* src/autofit/hbshim.c: Partially revert commit from 2014-04-17.

Using input glyph coverage data is simply wrong.

Problem reported by Nikolaus Waxweiler <madigens@gmail.com> and
Mantas Mikulėnas <grawity@gmail.com>.