Force software rendering when greeter crashed
We have a fallback QML theme for when the look and feel package is broken but when the
graphics driver is broken (e.g. proprietary graphics driver update) the lock screen would
stop working. When the greeter crashed, better play safe and force software rendering
Reviewed-By: d_ed
(cherry picked from commit 875e44f56d4597ba3b65f5bf4ff2b315376c05a4)