If two marks form a ligature, retain their previous lig_id, such that
the mark ligature can attach to ligature components...

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=676343

In fact, I noticed that we should not let ligatures form between glyphs
coming from different components of a previous ligature.  For example,
if the sequence is: LAM,SHADDA,LAM,FATHA,HEH, the LAM,LAM,HEH form a
ligature, putting SHADDA and FATHA next to eachother.  However, it would
be wrong to ligate them.  Uniscribe has this bug also.

Unlike Uniscribe, hb-old returns glyphs in logical order, so the logic
does not need to duplicated for RTL.

Wasn't a huge fan of putting the burden on the user.  Just remove it and
do what we've got to do transparently.

This commit: a3313e54 broke MarkMarkPos
when one of the marks itself is a ligature.  That regressed 26 Tibetan
tests (up from zero!).  Fix that.  Tibetan back to zero.

And use it to speed up the hotspot by checking coverage directly in
the main loop, not 10 functions deep in.

Gives me a solid 20% boost with Indic test suite.  Less so for less
lookup-intensive scenarios.

Remove the "fast_path" hack from before.

Should give us some performance boost.

Shapers have a chance to reject a font in face shaper_data creation.
No need to allow failing during planning.

Not optimized yet.  Eats babies.  And no shaper uses the shape_plan.

Add global shape_plan.  Unused so far.

Does not provide Uniscribe-compatible results, but should at least avoid
breaking hb-view due to out-of-order cluster values.

For RTL runs, ensure cluster values are non-increasing (instead of
non-decreasing).

Backporting from upstream:

commit b847f24ce855d24f6822bcd9c0006905e81b94d8
Author: Behdad Esfahbod <behdad@behdad.org>
Date:   Wed Jul 25 19:29:16 2012 -0400

    [arabic] Fix Arabic cursive positioning

    This was clearly broken in testing.  Who knows...  Fixes for me.
    Test with a Nastaleeq font, or with Arabic Typesetting.

    Backporting from Chromium.

Unlike its "documentation", hb-old's log_clusters are, well, indeed
logical, not visual.  Fixup.  Adapted / copied from hb-uniscribe.

This was broken as a result of 7b84c536.
As Khaled reported, MarkMark positioning was broken with glyphs
resulting from a MultipleSubst.  Fixed.  Test with the ALLAH character
in Amiri.

Choose using shaper name "old".