renderer: allow empty content during canvas rendering

This allow more generous handling of failure cases for invalid content,
thorvg can draw other successful contents.

Now, only raster engines can return fail cases in the picture.

src/renderer/tvgPicture.h

@@ -253,7 +253,7 @@ struct PictureImpl : Picture
 
     bool render(RenderMethod* renderer)
     {
-        bool ret = false;
+        auto ret = true;
         renderer->blend(impl.blendMethod);
 
         if (bitmap) return renderer->renderImage(impl.rd);