This works around a bug in QImage::isGrayScale where images with
16 bits/channel are not checked with allGray, so by default are not
grayscale. These images have depth 64, which is not in the switch
statement:

bool QImage::isGrayscale() const
{
    if (!d)
        return false;

    if (d->format == QImage::Format_Alpha8)
        return false;

    if (d->format == QImage::Format_Grayscale8)
        return true;

    switch (depth()) {
    case 32:
    case 24:
    case 16:
        return allGray();
    case 8: {
        Q_ASSERT(d->format == QImage::Format_Indexed8);
        for (int i = 0; i < colorCount(); i++)
            if (d->colortable.at(i) != qRgb(i,i,i))
                return false;
        return true;
        }
    }
    return false;
}

BUG:405693