[subset] never drop the 'pref' feature.

Never ever drop feature 'pref', even if it's empty. Harfbuzz uses it to choose the shaper for Khmer.

src/hb-ot-layout-gsubgpos.hh

@@ -3728,6 +3728,13 @@ struct GSUBGPOS
     for (unsigned i : feature_indices->iter())
     {
       const Feature& f = get_feature (i);
+      hb_tag_t tag =  get_feature_tag (i);
+      if (tag == HB_TAG ('p', 'r', 'e', 'f'))
+        // Note: Never ever drop feature 'pref', even if it's empty.
+        // HarfBuzz chooses shaper for Khmer based on presence of this
+        // feature.	See thread at:
+	// http://lists.freedesktop.org/archives/harfbuzz/2012-November/002660.html
+        continue;
 
       if (f.featureParams.is_null ()
 	  && !f.intersects_lookup_indexes (lookup_indices)

test/subset/data/Makefile.am

@@ -26,6 +26,7 @@ EXTRA_DIST += \
 	expected/layout.gsub5 \
 	expected/layout.gsub6 \
 	expected/layout.gsub8 \
+	expected/layout.khmer \
 	expected/layout.gdef \
 	expected/layout.context \
 	expected/layout.gdef-varstore \

test/subset/data/Makefile.sources

@@ -25,6 +25,7 @@ TESTS = \
 	tests/layout.gsub5.tests \
 	tests/layout.gsub6.tests \
 	tests/layout.gsub8.tests \
+	tests/layout.khmer.tests \
 	tests/layout.notonastaliqurdu.tests \
 	tests/layout.tests \
 	tests/sbix.tests \

test/subset/data/tests/layout.khmer.tests

@@ -0,0 +1,11 @@
+FONTS:
+Khmer.ttf
+
+PROFILES:
+keep-layout.txt
+keep-layout-retain-gids.txt
+
+SUBSETS:
+1
+ក
+ញុំបានមើ

test/subset/meson.build

@@ -19,6 +19,7 @@ tests = [
   'layout.gsub6',
   'layout.gsub8',
   'layout.gdef',
+  'layout.khmer',
   'layout.context',
   'layout.gdef-varstore',
   'layout.gdef-attachlist',