Controls: Avoid visual glitches in Material style

The ElevationEffect renders into a layer. However, as the layer
sampling was not specified as smooth, there was a risk that visual
glitches could be produced when the layer was rendered onto the scene.

Fixes: QTBUG-115759
Pick-to: 6.8
Change-Id: Id793fd224434b9b6e946ed4e31eba79e9f92db8e
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

src/quickcontrols/material/impl/ElevationEffect.qml

@@ -204,6 +204,7 @@ Item {
         // By rendering as a layer, the shadow will never show through the source item,
         // even when the source item's opacity is less than 1
         layer.enabled: true
+        layer.smooth: true
 
         // The box shadows automatically pick up the size of the source Item and not
         // the size of the parent, so we don't need to worry about the extra padding