[Icon Item] Make "smooth" property actually do something

We had m_smooth but never actually used it in the texture node.
With Plasma SVG icons this shouldn't be used but when passing custom pixmaps
or raster graphics (e.g. custom icon in Kicker, thumbnails in Folder View)

Differential Revision: https://phabricator.kde.org/D5214

src/declarativeimports/core/iconitem.cpp

@@ -462,6 +462,7 @@ QSGNode* IconItem::updatePaintNode(QSGNode *oldNode, UpdatePaintNodeData *update
         if (!textureNode || m_textureChanged) {
             delete oldNode;
             textureNode = new ManagedTextureNode;
+            textureNode->setFiltering(m_smooth ? QSGTexture::Linear : QSGTexture::Nearest);
             textureNode->setTexture(QSharedPointer<QSGTexture>(window()->createTextureFromImage(m_iconPixmap.toImage(), QQuickWindow::TextureCanUseAtlas)));
             m_sizeChanged = true;
             m_textureChanged = false;