lottie: support group-level blending

previously, only layer-level blending was supported.
Now, blending has been extended to support group-level as well.

src/loaders/lottie/tvgLottieBuilder.cpp

@@ -198,11 +198,18 @@ void LottieBuilder::updateGroup(LottieGroup* parent, LottieObject** child, float
     if (!group->visible) return;
 
     //Prepare render data
-    group->scene = parent->scene;
+    if (group->blendMethod == parent->blendMethod) {
+        group->scene = parent->scene;
+    } else {
+        group->scene = tvg::Scene::gen();
+        group->scene->blend(group->blendMethod);
+        parent->scene->push(group->scene);
+    }
+
     group->reqFragment |= ctx->reqFragment;
 
     //generate a merging shape to consolidate partial shapes into a single entity
-    if (group->mergeable()) _draw(parent, nullptr, ctx);
+    if (group->mergeable()) _draw(group, nullptr, ctx);
 
     Inlist<RenderContext> contexts;
     auto propagator = group->mergeable() ? ctx->propagator : static_cast<Shape*>(PAINT(ctx->propagator)->duplicate(group->pooling()));

src/loaders/lottie/tvgLottieModel.h

@@ -957,6 +957,7 @@ struct LottieGroup : LottieObject, LottieRenderPooler<tvg::Shape>
 
     Scene* scene = nullptr;
     Array<LottieObject*> children;
+    BlendMethod blendMethod = BlendMethod::Normal;
 
     bool reqFragment : 1;   //requirement to fragment the render context
     bool buildDone : 1;     //completed in building the composition.
@@ -1006,7 +1007,6 @@ struct LottieLayer : LottieGroup
     } cache;
 
     MaskMethod matteType = MaskMethod::None;
-    BlendMethod blendMethod = BlendMethod::Normal;
     Type type = Null;
     bool autoOrient = false;
     bool matteSrc = false;

src/loaders/lottie/tvgLottieParser.cpp

@@ -1079,6 +1079,8 @@ LottieObject* LottieParser::parseGroup()
         else if (KEY_AS("it")) {
             enterArray();
             while (nextArrayValue()) parseObject(group->children);
+        } else if (KEY_AS("bm")) {
+            group->blendMethod = (BlendMethod) getInt();
         } else skip();
     }
     group->prepare();