Unnecessary memcpy done by detach() of QImage::fill()

Unnecessary memcpy done by detach() of QImage::fill()

Profiling our application, we found that when QImage::fill() requires a detach - it unnecessarily performed a memcpy() (deep copy) of the image - unnecessary because fill() is about to overwrite the freshly copied image with new data.

(For example QByteArray::clear() is smart enough to not copy on detach).

Note: There is a workaround to avoid the memcpy - but users shouldn't have to be concerned with such thing.

        if (!m_Image.isDetached())
        {
            // avoid the unnecessary deep copy in detach because we are about to fill anyway.
            m_Image = QImage(m_Image.width(), m_Image.height(), m_Image.format());
        }
        m_Image.fill(0);