common canvas: ++ exception handling.

canvas update won't be performed if it's on drawing condition nor zero paints.

src/lib/tvgCanvasImpl.h

@@ -82,7 +82,7 @@ struct Canvas::Impl
 
     Result update(Paint* paint, bool force)
     {
-        if (!renderer) return Result::InsufficientCondition;
+        if (paints.count == 0 || drawing || !renderer) return Result::InsufficientCondition;
 
         Array<RenderData> clips;
         auto flag = RenderUpdateFlag::None;