widgets/cliprect: fix ShaderEffect warnings on reload

layer.effect causes warnings on reload for an unknown reason which
seems to be ownership or destruction time related. This commit uses
an alternate strategy to create the shader which does not show this
warning.

src/widgets/ClippingRectangle.qml

@@ -66,20 +66,22 @@ Item {
 	Item {
 		id: contentItemContainer
 		anchors.fill: root
+		layer.enabled: true
+		visible: false
 
 		Item {
 			id: contentItem
 			anchors.fill: parent
 			anchors.margins: root.contentInsideBorder ? root.border.width : 0
 		}
+	}
 
-		layer.enabled: true
-		layer.samplerName: "content"
-		layer.effect: ShaderEffect {
-			fragmentShader: `qrc:/Quickshell/Widgets/shaders/cliprect${root.contentUnderBorder ? "-ub" : ""}.frag.qsb`
-			property Rectangle rect: rectangle
-			property color backgroundColor: root.color
-			property color borderColor: root.border.color
-		}
+	ShaderEffect {
+		anchors.fill: contentItemContainer
+		fragmentShader: `qrc:/Quickshell/Widgets/shaders/cliprect${root.contentUnderBorder ? "-ub" : ""}.frag.qsb`
+		property Item content: contentItemContainer
+		property Rectangle rect: rectangle
+		property color backgroundColor: root.color
+		property color borderColor: root.border.color
 	}
 }