common: optimize scene composition

Scene could avoid composition if its children is only child (non scene)

This patch bring it conditionally check so as to avoid unnecesary expensive job.

@Issues: 254

src/lib/tvgPaint.h

@@ -26,9 +26,10 @@
 #include <math.h>
 #include "tvgRender.h"
 
-
 namespace tvg
 {
+    enum class PaintType { Shape = 0, Scene, Picture };
+
     struct StrategyMethod
     {
         virtual ~StrategyMethod() {}
@@ -52,6 +53,8 @@ namespace tvg
 
         uint8_t opacity = 255;
 
+        PaintType type;
+
         ~Impl() {
             if (cmpTarget) delete(cmpTarget);
             if (smethod) delete(smethod);

src/lib/tvgPicture.cpp

@@ -28,6 +28,7 @@
 
 Picture::Picture() : pImpl(new Impl(this))
 {
+    Paint::pImpl->type = PaintType::Picture;
     Paint::pImpl->method(new PaintMethod<Picture::Impl>(pImpl));
 }
 

src/lib/tvgScene.cpp

@@ -27,6 +27,7 @@
 
 Scene::Scene() : pImpl(new Impl())
 {
+    Paint::pImpl->type = PaintType::Scene;
     Paint::pImpl->method(new PaintMethod<Scene::Impl>(pImpl));
 }
 

src/lib/tvgSceneImpl.h

@@ -64,8 +64,14 @@ struct Scene::Impl
     {
         Compositor* cmp = nullptr;
 
+        //If scene has several children or only scene, it may require composition.
+        auto condition = false;
+        if ((paints.count > 1) || (paints.count == 1 && (*paints.data)->pImpl->type == PaintType::Scene)) {
+            condition = true;
+        }
+
         //Half translucent. This condition requires intermediate composition.
-        if ((opacity < 255 && opacity > 0) && (paints.count > 0)) {
+        if ((opacity < 255 && opacity > 0) && condition) {
             uint32_t x, y, w, h;
             if (!bounds(renderer, &x, &y, &w, &h)) return false;
             cmp = renderer.target(x, y, w, h);

src/lib/tvgShape.cpp

@@ -33,6 +33,7 @@ constexpr auto PATH_KAPPA = 0.552284f;
 
 Shape :: Shape() : pImpl(new Impl(this))
 {
+    Paint::pImpl->type = PaintType::Shape;
     Paint::pImpl->method(new PaintMethod<Shape::Impl>(pImpl));
 }