lottie: optimize the rendering by preventing partial rendering branches.

this optimization is to figure out if this group is a simple path drawing.
In that case, the rendering context can be sharable with the parent's.

src/loaders/lottie/tvgLottieModel.cpp

@@ -142,13 +142,20 @@ void LottieGroup::prepare(LottieObject::Type type)
     size_t fillCnt = 0;
 
     for (auto c = children.end() - 1; c >= children.begin(); --c) {
-        if (reqFragment) break;
+        if (!mergeable && reqFragment) break;
+
         auto child = static_cast<LottieObject*>(*c);
+
+        /* Figure out if this group is a simple path drawing.
+           In that case, the rendering context can be sharable with the parent's. */
+        if (mergeable && !child->mergeable()) mergeable = false;
+
+        if (reqFragment) continue;
+
         /* Figure out if the rendering context should be fragmented.
            Multiple stroking or grouping with a stroking would occur this.
            This fragment resolves the overlapped stroke outlines. */
-        if (reqFragment) continue;
-        if (child->type == LottieObject::Group) {
+        if (child->type == LottieObject::Group && !static_cast<LottieGroup*>(child)->mergeable) {
             if (strokeCnt > 0 || fillCnt > 0) reqFragment = true;
         } else if (child->type == LottieObject::SolidStroke || child->type == LottieObject::GradientStroke) {
             if (strokeCnt > 0) reqFragment = true;

src/loaders/lottie/tvgLottieModel.h

@@ -124,6 +124,8 @@ struct LottieObject
         TVGERR("LOTTIE", "Unsupported slot type");
     }
 
+    virtual bool mergeable() { return false; }
+
     char* name = nullptr;
     Type type;
     bool hidden = false;       //remove?
@@ -195,17 +197,23 @@ struct LottieText : LottieObject
 
 struct LottieTrimpath : LottieObject
 {
-    enum Type : uint8_t { Individual = 1, Simultaneous = 2 };
+    enum Type : uint8_t { Simultaneous = 1, Individual = 2 };
 
     void prepare()
     {
         LottieObject::type = LottieObject::Trimpath;
     }
 
+    bool mergeable() override
+    {
+        if (!start.frames && start.value == 0.0f && !end.frames && end.value == 100.0f && !offset.frames && offset.value == 0.0f) return true;
+        return false;
+    }
+
     void segment(float frameNo, float& start, float& end);
 
     LottieFloat start = 0.0f;
-    LottieFloat end = 0.0f;
+    LottieFloat end = 100.0f;
     LottieFloat offset = 0.0f;
     Type type = Simultaneous;
 };
@@ -215,6 +223,11 @@ struct LottieShape : LottieObject
 {
     virtual ~LottieShape() {}
     uint8_t direction = 0;   //0: clockwise, 2: counter-clockwise, 3: xor(?)
+
+    bool mergeable() override
+    {
+        return true;
+    }
 };
 
 
@@ -309,6 +322,12 @@ struct LottieTransform : LottieObject
         LottieObject::type = LottieObject::Transform;
     }
 
+    bool mergeable() override
+    {
+        if (!opacity.frames && opacity.value == 255) return true;
+        return false;
+    }
+
     LottiePosition position = Point{0.0f, 0.0f};
     LottieFloat rotation = 0.0f;           //z rotation
     LottiePoint scale = Point{100.0f, 100.0f};
@@ -559,6 +578,7 @@ struct LottieGroup : LottieObject
 
     bool reqFragment = false;   //requirment to fragment the render context
     bool buildDone = false;     //completed in building the composition.
+    bool mergeable = true;     //if this group is consisted of simple (transformed) shapes.
 };