[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%)

src/hb-ot-layout-gsubgpos-private.hh

@@ -1066,9 +1066,11 @@ static inline bool chain_context_would_apply_lookup (hb_would_apply_context_t *c
 						     const LookupRecord lookupRecord[],
 						     ChainContextApplyLookupContext &lookup_context)
 {
-  return !backtrackCount
-      && !lookaheadCount
-      && would_match_input (c,
+  /* 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.  Hence the commented out line. */
+  return /* !backtrackCount && !lookaheadCount && */
+         would_match_input (c,
 			    inputCount, input,
 			    lookup_context.funcs.match, lookup_context.match_data[1]);
 }