[beyond-64k/subset] Fetch lsb from glyph table if not available

The beyond-64k hmtx table doesn't encode LSB. If subsetting brings
the glyph under 64k (which currently is the only mode we support),
then we need to encode the LSB, which wasn't available. We need to
fetch xMin from glyf table and set it as LSB.

src/OT/glyf/Glyph.hh

@@ -515,6 +515,7 @@ struct Glyph
 
   hb_bytes_t get_bytes () const { return bytes; }
   glyph_type_t get_type () const { return type; }
+  const GlyphHeader *get_header () const { return header; }
 
   Glyph () : bytes (),
              header (bytes.as<GlyphHeader> ()),

src/OT/glyf/glyf.hh

@@ -346,6 +346,15 @@ struct glyf_accelerator_t
   }
 #endif
 
+  bool get_leading_bearing_without_var_unscaled (hb_codepoint_t gid, bool is_vertical, int *lsb) const
+  {
+    if (unlikely (gid >= num_glyphs)) return false;
+    if (is_vertical) return false; // TODO Humm, what to do here?
+
+    *lsb = glyph_for_gid (gid).get_header ()->xMin;
+    return true;
+  }
+
   public:
   bool get_extents (hb_font_t *font, hb_codepoint_t gid, hb_glyph_extents_t *extents) const
   {

src/hb-ot-font.cc

@@ -633,20 +633,4 @@ hb_ot_font_set_funcs (hb_font_t *font)
 		     _hb_ot_font_destroy);
 }
 
-#ifndef HB_NO_VAR
-bool
-_glyf_get_leading_bearing_with_var_unscaled (hb_font_t *font, hb_codepoint_t glyph, bool is_vertical,
-					     int *lsb)
-{
-  return font->face->table.glyf->get_leading_bearing_with_var_unscaled (font, glyph, is_vertical, lsb);
-}
-
-unsigned
-_glyf_get_advance_with_var_unscaled (hb_font_t *font, hb_codepoint_t glyph, bool is_vertical)
-{
-  return font->face->table.glyf->get_advance_with_var_unscaled (font, glyph, is_vertical);
-}
-#endif
-
-
 #endif

src/hb-ot-hmtx-table.hh

@@ -50,6 +50,9 @@ _glyf_get_leading_bearing_with_var_unscaled (hb_font_t *font, hb_codepoint_t gly
 HB_INTERNAL unsigned
 _glyf_get_advance_with_var_unscaled (hb_font_t *font, hb_codepoint_t glyph, bool is_vertical);
 
+HB_INTERNAL bool
+_glyf_get_leading_bearing_without_var_unscaled (hb_face_t *face, hb_codepoint_t gid, bool is_vertical, int *lsb);
+
 
 namespace OT {
 
@@ -208,7 +211,8 @@ struct hmtxvmtx
 		  if (!c->plan->old_gid_for_new_gid (_, &old_gid))
 		    return hb_pair (0u, 0);
 		  int lsb = 0;
-		  (void) _mtx.get_leading_bearing_without_var_unscaled (old_gid, &lsb);
+		  if (!_mtx.get_leading_bearing_without_var_unscaled (old_gid, &lsb))
+		    (void) _glyf_get_leading_bearing_without_var_unscaled (c->plan->source, old_gid, !T::is_horizontal, &lsb);
 		  return hb_pair (_mtx.get_advance_without_var_unscaled (old_gid), +lsb);
 		}
 		return mtx_map->get (_);

src/hb-static.cc

@@ -36,6 +36,7 @@
 #include "OT/Color/COLR/COLR.hh"
 #include "hb-ot-glyf-table.hh"
 #include "hb-ot-head-table.hh"
+#include "hb-ot-hmtx-table.hh"
 #include "hb-ot-maxp-table.hh"
 
 #ifndef HB_NO_VISIBILITY
@@ -108,4 +109,26 @@ hb_face_t::load_upem () const
 }
 
 
+#ifndef HB_NO_VAR
+bool
+_glyf_get_leading_bearing_with_var_unscaled (hb_font_t *font, hb_codepoint_t glyph, bool is_vertical,
+					     int *lsb)
+{
+  return font->face->table.glyf->get_leading_bearing_with_var_unscaled (font, glyph, is_vertical, lsb);
+}
+
+unsigned
+_glyf_get_advance_with_var_unscaled (hb_font_t *font, hb_codepoint_t glyph, bool is_vertical)
+{
+  return font->face->table.glyf->get_advance_with_var_unscaled (font, glyph, is_vertical);
+}
+#endif
+
+bool
+_glyf_get_leading_bearing_without_var_unscaled (hb_face_t *face, hb_codepoint_t gid, bool is_vertical, int *lsb)
+{
+  return face->table.glyf->get_leading_bearing_without_var_unscaled (gid, is_vertical, lsb);
+}
+
+
 #endif