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".

Results match Uniscribe now.

Does not attempt to handle clusters in a Uniscribe- or HarfBuzz-compatible way;
just returns the original string indexes that CT maintains. These may even be
out-of-order in the case of reordrant glyphs.

Should fix warnings generated when building with -Wunused-private-field.
Based on patch from Jonathan Kew.

The font is forming a post-base consonant in some samples, and Uniscribe
positions top matra on the post-base.  Do the same.

Gurmukhi failures down from 59 to 41 (0.0674242%).

Uniscribe is buggy and sometimes /eats/ a mark next to a non-joiner.
Most of Malayalam failures where actually hitting this bug.

Ignore test output with two zero-width space glyphs.  This is a hack
until we build up the test suite infrastructure better.

Bengali went down by 9, Devanagari by 2, Kannada by 130, Malayalm down
from 1197 to 307, Sinhala down by 16, Telugu down by 26.  New stats:

BENGALI: 353996 out of 354285 tests passed. 289 failed (0.0815727%)
DEVANAGARI: 693573 out of 693628 tests passed. 55 failed (0.00792932%)
GUJARATI: 366489 out of 366506 tests passed. 17 failed (0.0046384%)
GURMUKHI: 60750 out of 60809 tests passed. 59 failed (0.0970251%)
KANNADA: 951086 out of 951913 tests passed. 827 failed (0.0868777%)
KHMER: 299094 out of 299124 tests passed. 30 failed (0.0100293%)
MALAYALAM: 1048109 out of 1048416 tests passed. 307 failed (0.0292823%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271715 out of 271847 tests passed. 132 failed (0.0485567%)
TAMIL: 1091837 out of 1091837 tests passed. 0 failed (0%)
TELUGU: 970550 out of 970573 tests passed. 23 failed (0.00236973%)

Just put it before base, which is what's expected.

Malayalam failures down from 1559 to 1197 (0.114172%).

BENGALI: 353988 out of 354285 tests passed. 297 failed (0.0838308%)
DEVANAGARI: 693571 out of 693628 tests passed. 57 failed (0.00821766%)
GUJARATI: 366489 out of 366506 tests passed. 17 failed (0.0046384%)
GURMUKHI: 60750 out of 60809 tests passed. 59 failed (0.0970251%)
KANNADA: 950956 out of 951913 tests passed. 957 failed (0.100534%)
KHMER: 299094 out of 299124 tests passed. 30 failed (0.0100293%)
MALAYALAM: 1047219 out of 1048416 tests passed. 1197 failed (0.114172%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271699 out of 271847 tests passed. 148 failed (0.0544424%)
TAMIL: 1091837 out of 1091837 tests passed. 0 failed (0%)
TELUGU: 970524 out of 970573 tests passed. 49 failed (0.00504856%)