[windowthumbnails] Guard against software QtQuick backend
If we are using the software renderer QQuickWindow::createTextureFromNativeObject returns null. We already checking for kwin using GL compositing so we can expand to check QtQuick's usage.
Fixes https://errors-eval.kde.org/organizations/kde/issues/195 I have no verification they were using the QtQuick software backend, but it's the only thing that makes sense.
Testing done: export QMLSCENE_DEVICE=softwarecontext
As follow ups we should also:
- fix the placeholder image fallback path. It currently breaks as we have an early return if there's no offscreen texture
- make QuickSceneEffect::supported() also check the scene graph backend, unfortunately we can't query the backend until the first QQuickWindow is made.