For software QtQuick rendering we want to create a pixmap buffer and
render our contents in there.
m_rendercontrol->grab does this, but also covers the GL side in pretty
much the same way that we were currently doing.
Some calls are redundant in software mode, but they don't do any harm,
and I didn't want to pollute the code.
In doing so I found a bug in Qt, that's submitted upstream, and also worked round.