Insufficient detection of software rasterizer (opengl32sw.dll)

Coin tries to switch to using opengl32sw.dll on Windows. Anyhow, this crashes the qmltest of Qt Canvas3D crash:

https://codereview.qt-project.org/#/c/170519/

Interestingly the test succeeds if the software rasterizer is enforced by the Qt::AA_UseSoftwareOpenGL application attribute, instead of the QT_OPENGL environment variable or automatic selection. This is because the attribute is detected explicitly in canvas3d.cpp:

// ...
    if (QCoreApplication::testAttribute(Qt::AA_UseSoftwareOpenGL))
        m_isSoftwareRendered = true;
// ...
        if (m_maxSamples == 0 || m_isSoftwareRendered)
            m_contextAttribs.setAntialias(false);