common interator: add count() interface.

src/lib/tvgPaint.h

@@ -31,6 +31,7 @@ namespace tvg
     {
         virtual ~Iterator() {}
         virtual const Paint* next() = 0;
+        virtual uint32_t count() = 0;
     };
 
     struct StrategyMethod

src/lib/tvgPictureImpl.h

@@ -42,6 +42,12 @@ struct PictureIterator : Iterator
         paint = nullptr;
         return ret;
     }
+
+    uint32_t count() override
+    {
+        if (paint) return 1;
+        else return 0;
+    }
 };
 
 

src/lib/tvgSceneImpl.h

@@ -43,6 +43,11 @@ struct SceneIterator : Iterator
         if (idx >= paints->count) return nullptr;
         return paints->data[idx++];
     }
+
+    uint32_t count() override
+    {
+        return paints->count;
+    }
 };
 
 struct Scene::Impl