Fix glyph closure for alternate substitution.

It was not filtering on the glyphs to be retained.

src/hb-ot-layout-gsub-table.hh

@@ -605,6 +605,7 @@ struct AlternateSubstFormat1
   void closure (hb_closure_context_t *c) const
   {
     + hb_zip (this+coverage, alternateSet)
+    | hb_filter(c->glyphs, hb_first)
     | hb_map (hb_second)
     | hb_map (hb_add (this))
     | hb_apply ([c] (const AlternateSet &_) { _.closure (c); })

test/subset/data/Makefile.am

@@ -18,6 +18,7 @@ EXTRA_DIST += \
 	expected/layout.gpos3 \
 	expected/layout.gpos4 \
 	expected/layout.gpos6 \
+	expected/layout.gsub3 \
 	expected/layout.gsub6 \
 	expected/cmap \
 	expected/cmap14 \

test/subset/data/Makefile.sources

@@ -10,6 +10,7 @@ TESTS = \
 	tests/layout.gpos3.tests \
 	tests/layout.gpos4.tests \
 	tests/layout.gpos6.tests \
+	tests/layout.gsub3.tests \
 	tests/layout.gsub6.tests \
 	tests/cmap.tests \
 	tests/cmap14.tests \

test/subset/data/tests/layout.gsub3.tests

@@ -0,0 +1,12 @@
+FONTS:
+gsub_alternate_substitution.otf
+
+PROFILES:
+keep-layout.txt
+keep-layout-retain-gids.txt
+
+SUBSETS:
+厩
+叱
+厩叱
+*