[COLR] Implement paint templates

Based on googlefonts/colr-gradients-spec#371

See https://github.com/behdad/colr-templates for usage.

src/OT/Color/COLR/COLR.hh

@@ -49,6 +49,7 @@ namespace OT {
 struct COLR;
 
 struct Paint;
+struct PaintTemplateInstance;
 
 struct hb_paint_context_t :
        hb_dispatch_context_t<hb_paint_context_t>
@@ -68,6 +69,7 @@ public:
   unsigned int palette_index;
   hb_color_t foreground;
   VarStoreInstancer &instancer;
+  hb_vector_t<const PaintTemplateInstance *> template_stack;
   int depth_left = HB_MAX_NESTING_LEVEL;
   int edge_count = HB_COLRV1_MAX_EDGE_COUNT;
 
@@ -1454,6 +1456,72 @@ struct PaintComposite
   DEFINE_SIZE_STATIC (8);
 };
 
+struct PaintTemplateInstance
+{
+  void closurev1 (hb_colrv1_closure_context_t* c) const;
+
+  bool subset (hb_subset_context_t *c,
+               const VarStoreInstancer &instancer) const
+  {
+    TRACE_SUBSET (this);
+    // XXX
+    //auto *out = c->serializer->embed (this);
+    //return_trace (out);
+    return_trace (false);
+  }
+
+  bool sanitize (hb_sanitize_context_t *c) const
+  {
+    TRACE_SANITIZE (this);
+    return_trace (c->check_struct (this));
+  }
+
+  void paint_glyph (hb_paint_context_t *c) const
+  {
+    c->template_stack.push (this);
+    c->recurse (this+templatePaint);
+    c->template_stack.pop ();
+  }
+
+  HBUINT8		format; /* format = 33 */
+  Offset24To<Paint>	templatePaint; /* Offset (from beginning of PaintTemplateInstance table) to Paint subtable. */
+  ArrayOf<Offset24To<Paint>, HBUINT8>
+			arguments; /* Array of offsets (from beginning of PaintTemplateInstance table) to Paint subtables. */
+  public:
+  DEFINE_SIZE_ARRAY (5, arguments);
+};
+
+struct PaintTemplateArgument
+{
+  void closurev1 (hb_colrv1_closure_context_t* c) const {}
+
+  bool subset (hb_subset_context_t *c,
+               const VarStoreInstancer &instancer) const
+  {
+    TRACE_SUBSET (this);
+    auto *out = c->serializer->embed (this);
+    return_trace (out);
+  }
+
+  bool sanitize (hb_sanitize_context_t *c) const
+  {
+    TRACE_SANITIZE (this);
+    return_trace (c->check_struct (this));
+  }
+
+  void paint_glyph (hb_paint_context_t *c) const
+  {
+    auto *templatePaint = c->template_stack.tail ();
+    if (!templatePaint) return;
+    c->recurse (templatePaint+templatePaint->arguments[index]);
+  }
+
+  HBUINT8		format; /* format = 34 */
+  HBUINT8		index;
+  public:
+  DEFINE_SIZE_STATIC (2);
+};
+
 struct ClipBoxData
 {
   int xMin, yMin, xMax, yMax;
@@ -1780,6 +1848,8 @@ struct Paint
     case 30: return_trace (c->dispatch (u.paintformat30, std::forward<Ts> (ds)...));
     case 31: return_trace (c->dispatch (u.paintformat31, std::forward<Ts> (ds)...));
     case 32: return_trace (c->dispatch (u.paintformat32, std::forward<Ts> (ds)...));
+    case 33: return_trace (c->dispatch (u.paintformat33, std::forward<Ts> (ds)...));
+    case 34: return_trace (c->dispatch (u.paintformat34, std::forward<Ts> (ds)...));
     default:return_trace (c->default_return_value ());
     }
   }
@@ -1819,6 +1889,8 @@ struct Paint
   NoVariable<PaintSkewAroundCenter>		paintformat30;
   Variable<PaintSkewAroundCenter>		paintformat31;
   PaintComposite				paintformat32;
+  PaintTemplateInstance				paintformat33;
+  PaintTemplateArgument				paintformat34;
   } u;
   public:
   DEFINE_SIZE_MIN (2);

src/OT/Color/COLR/colrv1-closure.hh

@@ -100,6 +100,10 @@ HB_INTERNAL void PaintComposite::closurev1 (hb_colrv1_closure_context_t* c) cons
   (this+src).dispatch (c);
   (this+backdrop).dispatch (c);
 }
+HB_INTERNAL void PaintTemplateInstance::closurev1 (hb_colrv1_closure_context_t* c) const
+{
+  (this+templatePaint).dispatch (c);
+}
 
 } /* namespace OT */