Whitespace
[Indic] Only apply basic features per-syllable

Free up syllables and let features work across syllables for the
presentation forms features and GPOS.

Fixed:
- 1 GURMUKHI test (remains 40)
- 12 KHMER tests (remains 18)
- 11 SINHALA tests (remains 121)

Regresses:
- 5 MALAYALAM tests (up to 312)

Current numbers:

BENGALI: 353996 out of 354285 tests passed. 289 failed (0.0815727%)
DEVANAGARI: 707339 out of 707394 tests passed. 55 failed (0.00777502%)
GUJARATI: 366489 out of 366506 tests passed. 17 failed (0.0046384%)
GURMUKHI: 60769 out of 60809 tests passed. 40 failed (0.0657797%)
KANNADA: 951086 out of 951913 tests passed. 827 failed (0.0868777%)
KHMER: 299106 out of 299124 tests passed. 18 failed (0.00601757%)
LAO: 53611 out of 53644 tests passed. 33 failed (0.0615167%)
MALAYALAM: 1048104 out of 1048416 tests passed. 312 failed (0.0297592%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271726 out of 271847 tests passed. 121 failed (0.0445103%)
TAMIL: 1091837 out of 1091837 tests passed. 0 failed (0%)
TELUGU: 970558 out of 970573 tests passed. 15 failed (0.00154548%)
TIBETAN: 208469 out of 208469 tests passed. 0 failed (0%)
[Indic] Implement pre-base reordering Ra for old-spec Malayalam

Fixes Pa,H,Ra sequence with AnjaliNewLipi.ttf.
[Indic/GSUB] Ignore context when matching would_apply()

The MS Indic specs say "...all classifications are determined ... using
context-free substitutions."  However, testing shows that MS's Malayalam
shapers (both old and new), "match" even if there is no zero-context rule.
We follow.

Fixes below-base La (eg. Pa,H,La) with AnjaliNewLipi.ttf (old spec).
Moreover, test suite Malayalam failures are down to 312 from 875!  No
change in other scripts.

Current numbers:

BENGALI: 353996 out of 354285 tests passed. 289 failed (0.0815727%)
DEVANAGARI: 707339 out of 707394 tests passed. 55 failed (0.00777502%)
GUJARATI: 366489 out of 366506 tests passed. 17 failed (0.0046384%)
GURMUKHI: 60769 out of 60809 tests passed. 40 failed (0.0657797%)
KANNADA: 951086 out of 951913 tests passed. 827 failed (0.0868777%)
KHMER: 299106 out of 299124 tests passed. 18 failed (0.00601757%)
LAO: 53611 out of 53644 tests passed. 33 failed (0.0615167%)
MALAYALAM: 1047541 out of 1048416 tests passed. 875 failed (0.0834592%)
ORIYA: 42320 out of 42329 tests passed. 9 failed (0.021262%)
SINHALA: 271726 out of 271847 tests passed. 121 failed (0.0445103%)
TAMIL: 1091837 out of 1091837 tests passed. 0 failed (0%)
TELUGU: 970558 out of 970573 tests passed. 15 failed (0.00154548%)
TIBETAN: 208469 out of 208469 tests passed. 0 failed (0%)
Revert "[Indic/GSUB] Ignore context when matching would_apply()"

This reverts commit 24dd4e56743c6ce5e01cb710ca9e01b3e527af58.

Oops.  My bad.  The change _regressed_ Malayalam test suite, not
improved it.  I'll redo it, differentiating between old-spec and
new-spec cases.
[GSUB] Allow non-zero-context matching in would_apply()

To be used in the next patch.
[Indic] For old-spec, match non-zero context

Fixes consonant-position with old-spec Malayalam.  Uniscribe seem to be
doing this.  Fixes below-base La (eg. Pa,H,La) with AnjaliNewLipi.ttf.
Doesn't regress new-spec or other scripts.
[icu-le] Start adding a icu-layout-engine backend

Import PortableFontInstance and add shaper stub.
[icu-le] Actually use the FontTableCache
[icu-le] Hook up to hb_face_t
[icu-le] Implement icu layout engine shaper
[graphite2] Cleanup scratch buffer allocation
Remove TODO items
Bug 53815 - CoreText backend doesn't link
[Indic] Fix switch

D'oh.  Was working by pure chance :)).
Minor
Center unknown marks horizontally
Only fallback-position glyphs if we have the ccc

Previously, ccc=0 Thai / Lao marks were being
mispositioned.  Don't touch them.
Minor
Minor