plugins/overview: Replace OpacityMask with ShadowedTexture

The OpacityMask is used purely for rounding the corners. We can get rid
of it by using ShadowedTexture which does the same thing while also
drawing the shadow, meaning we can also eliminate the ShadowedRectangle.

src/plugins/overview/qml/main.qml

@@ -409,9 +409,9 @@ FocusScope {
             property Item currentHeap
             property Item currentBackgroundItem
 
-            Kirigami.ShadowedRectangle {
+            Item {
                 id: mainBackground
-                color: Kirigami.Theme.highlightColor
+
                 visible: gridVal > 0 || nearCurrent
                 anchors.fill: parent
                 property bool shouldBeVisibleInOverview: !(effect.searchText.length > 0 && current) || (heap.count !== 0 && effect.filterWindows)
@@ -431,13 +431,6 @@ FocusScope {
                 z: dragActive ? 1 : 0
                 readonly property bool dragActive: heap.dragActive || dragHandler.active
 
-                shadow {
-                    size: Kirigami.Units.gridUnit * 2
-                    color: Qt.rgba(0, 0, 0, 0.3)
-                    yOffset: 3
-                }
-                radius: Kirigami.Units.largeSpacing * 2 * (overviewVal + gridVal * 2)
-
                 property int gridSize: Math.max(rows, columns)
                 property real row: (index - column) / columns
                 property real column: index % columns
@@ -524,14 +517,19 @@ FocusScope {
                     outputName: targetScreen.name
                     visible: false
                 }
-                OpacityMask {
+
+                Kirigami.ShadowedTexture {
                     anchors.fill: parent
+
+                    color: Kirigami.Theme.highlightColor
                     source: desktopElement
-                    maskSource: Rectangle {
-                        anchors.centerIn: parent
-                        width: desktopElement.width
-                        height: desktopElement.height
-                        radius: mainBackground.radius
+
+                    radius: Kirigami.Units.largeSpacing * 2 * (overviewVal + gridVal * 2)
+
+                    shadow {
+                        size: Kirigami.Units.gridUnit * 2
+                        color: Qt.rgba(0, 0, 0, 0.3)
+                        yOffset: 3
                     }
                 }