scripting: Make thumbnaills use linear filter

QSGImageNode ignores QSGTexture's filter, one has to specify it using
QSGImageNode::setFiltering().

In hindsight, we need to figure out how to integrate the lanczos filter
with window thumbnails and use it instead.

BUG: 441709

src/scripting/thumbnailitem.cpp

@@ -229,6 +229,7 @@ QSGNode *ThumbnailItemBase::updatePaintNode(QSGNode *oldNode, QQuickItem::Update
     QSGImageNode *node = static_cast<QSGImageNode *>(oldNode);
     if (!node) {
         node = window()->createImageNode();
+        node->setFiltering(QSGTexture::Linear);
     }
     node->setTexture(m_provider->texture());