[subset] Add morx, mort, kern, and kernx to the default layout tables drop list.

src/hb-subset-input.cc

@@ -56,6 +56,10 @@ hb_subset_input_create_or_fail ()
     HB_TAG ('G', 'S', 'U', 'B'),
     HB_TAG ('G', 'P', 'O', 'S'),
     HB_TAG ('G', 'D', 'E', 'F'),
+    HB_TAG ('m', 'o', 'r', 'x'),
+    HB_TAG ('m', 'o', 'r', 't'),
+    HB_TAG ('k', 'e', 'r', 'x'),
+    HB_TAG ('k', 'e', 'r', 'n'),
 
     // Copied from fontTools:
     HB_TAG ('B', 'A', 'S', 'E'),

src/hb-subset.cc

@@ -254,6 +254,10 @@ _should_drop_table (hb_subset_plan_t *plan, hb_tag_t tag)
     case HB_OT_TAG_GDEF:
     case HB_OT_TAG_GPOS:
     case HB_OT_TAG_GSUB:
+    case HB_TAG ('m', 'o', 'r', 'x'):
+    case HB_TAG ('m', 'o', 'r', 't'):
+    case HB_TAG ('k', 'e', 'r', 'x'):
+    case HB_TAG ('k', 'e', 'r', 'n'):
       return true;
 #endif