[GPOS] If an Anchor offset is NULL, return false

If in a MarkPos table, a base has no anchor for a particular mark class,
return NULL such that the subsequent subtables get a chance at it.

Test case:
hb-shape ./EBGaramond12-Regular.otf ἂ --features="ss20","smcp"

src/hb-open-type-private.hh

@@ -616,10 +616,20 @@ struct Index : USHORT {
 DEFINE_NULL_DATA (Index, "\xff\xff");
 
 /* Offset to a table, same as uint16 (length = 16 bits), Null offset = 0x0000 */
-typedef USHORT Offset;
+struct Offset : USHORT
+{
+  inline bool is_null (void) const { return 0 == *this; }
+  public:
+  DEFINE_SIZE_STATIC (2);
+};
 
 /* LongOffset to a table, same as uint32 (length = 32 bits), Null offset = 0x00000000 */
-typedef ULONG LongOffset;
+struct LongOffset : ULONG
+{
+  inline bool is_null (void) const { return 0 == *this; }
+  public:
+  DEFINE_SIZE_STATIC (4);
+};
 
 
 /* CheckSum */

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

@@ -336,8 +336,10 @@ struct Anchor
 
 struct AnchorMatrix
 {
-  inline const Anchor& get_anchor (unsigned int row, unsigned int col, unsigned int cols) const {
+  inline const Anchor& get_anchor (unsigned int row, unsigned int col, unsigned int cols, bool *found) const {
+    *found = false;
     if (unlikely (row >= rows || col >= cols)) return Null(Anchor);
+    *found = !matrix[row * cols + col].is_null ();
     return this+matrix[row * cols + col];
   }
 
@@ -392,7 +394,11 @@ struct MarkArray : ArrayOf<MarkRecord>	/* Array of MarkRecords--in Coverage orde
     unsigned int mark_class = record.klass;
 
     const Anchor& mark_anchor = this + record.markAnchor;
-    const Anchor& glyph_anchor = anchors.get_anchor (glyph_index, mark_class, class_count);
+    bool found;
+    const Anchor& glyph_anchor = anchors.get_anchor (glyph_index, mark_class, class_count, &found);
+    /* If this subtable doesn't have an anchor for this base and this class,
+     * return false such that the subsequent subtables have a chance at it. */
+    if (unlikely (!found)) return TRACE_RETURN (false);
 
     hb_position_t mark_x, mark_y, base_x, base_y;