Function knobDragStarted

dgl/src/ImageBaseWidgets.cpp:295–300

    }

    void knobDragStarted(SubWidget* const widget) override
    {
        if (callback != nullptr)
            if (ImageBaseKnob* const imageKnob = dynamic_cast<ImageBaseKnob*>(widget))
                callback->imageKnobDragStarted(imageKnob);
    }

    void knobDragFinished(SubWidget* const widget) override
    {