[indic] Handle old-spec Malayalam reordering with final Halant

See comment.

Micro-tests added.

src/hb-ot-shape-complex-indic.cc

@@ -921,14 +921,32 @@ initial_reordering_consonant_syllable (const hb_ot_shape_plan_t *plan,
     info[start].indic_position() = POS_RA_TO_BECOME_REPH;
 
   /* For old-style Indic script tags, move the first post-base Halant after
-   * last consonant.  Only do this if there is *not* a Halant after last
-   * consonant.  Otherwise it becomes messy. */
-  if (indic_plan->is_old_spec) {
+   * last consonant.
+   *
+   * Reports suggest that in some scripts Uniscribe does this only if there
+   * is *not* a Halant after last consonant already (eg. Kannada), while it
+   * does it unconditionally in other scripts (eg. Malayalam).  We don't
+   * currently know about other scripts, so we single out Malayalam for now.
+   *
+   * Kannada test case:
+   * U+0C9A,U+0CCD,U+0C9A,U+0CCD
+   * With some versions of Lohit Kannada.
+   * https://bugs.freedesktop.org/show_bug.cgi?id=59118
+   *
+   * Malayalam test case:
+   * U+0D38,U+0D4D,U+0D31,U+0D4D,U+0D31,U+0D4D
+   * With lohit-ttf-20121122/Lohit-Malayalam.ttf
+   */
+  if (indic_plan->is_old_spec)
+  {
+    bool disallow_double_halants = buffer->props.script != HB_SCRIPT_MALAYALAM;
     for (unsigned int i = base + 1; i < end; i++)
-      if (info[i].indic_category() == OT_H) {
+      if (info[i].indic_category() == OT_H)
+      {
         unsigned int j;
         for (j = end - 1; j > i; j--)
-	  if (is_consonant (info[j]) || info[j].indic_category() == OT_H)
+	  if (is_consonant (info[j]) ||
+	      (disallow_double_halants && info[j].indic_category() == OT_H))
 	    break;
 	if (info[j].indic_category() != OT_H && j > i) {
 	  /* Move Halant to after last consonant. */

test/shaping/Makefile.am

@@ -38,6 +38,7 @@ CLEANFILES += \
 TESTS = \
 	tests/arabic-feature-order.tests \
 	tests/context-matching.tests \
+	tests/indic-old-spec.tests \
 	tests/indic-pref-blocking.tests \
 	tests/mongolian-variation-selector.tests \
 	$(NULL)

test/shaping/fonts/sha1sum/MANIFEST

 226bc2deab3846f1a682085f70c67d0421014144.ttf
+270b89df543a7e48e206a2d830c0e10e5265c630.ttf
 37033cc5cf37bb223d7355153016b6ccece93b28.ttf
 4cce528e99f600ed9c25a2b69e32eb94a03b4ae8.ttf
+57a9d9f83020155cbb1d2be1f43d82388cbecc88.ttf
 813c2f8e5512187fd982417a7fb4286728e6f4a8.ttf
 8a9fea2a7384f2116e5b84a9b31f83be7850ce21.ttf
 a919b33197965846f21074b24e30250d67277bce.ttf

test/shaping/tests/MANIFEST

 arabic-feature-order.tests
 context-matching.tests
+indic-old-spec.tests
 indic-pref-blocking.tests
 mongolian-variation-selector.tests

test/shaping/tests/indic-old-spec.tests

+fonts/sha1sum/57a9d9f83020155cbb1d2be1f43d82388cbecc88.ttf:U+0C9A,U+0CCD,U+0C9A,U+0CCD:[U0C9A_U0CCD.haln=0+1066|U0C9A_0CCD.blwf=0+0]
+fonts/sha1sum/270b89df543a7e48e206a2d830c0e10e5265c630.ttf:U+0D38,U+0D4D,U+0D31,U+0D4D,U+0D31,U+0D4D:[glyph201=0+1183|U0D4D=0+0]

test/shaping/texts/in-tree/shaper-indic/indic/script-malayalam/misc/misc.txt

@@ -62,3 +62,4 @@
 ശിം‌
 കോം‌
 യ‍്യ
+സ്റ്റ്