previewRenderer: add rounded corner by using OpacityMask

Change-Id: I4433a72af9c5da9cb905cfcb742f71d53427207a

src/mainview/components/VideoCallPage.qml

@@ -319,6 +319,15 @@ Rectangle {
                         }
                     }
 
+                    layer.enabled: true
+                    layer.effect: OpacityMask {
+                        maskSource: Rectangle {
+                            width: previewRenderer.width
+                            height: previewRenderer.height
+                            radius: JamiTheme.lineEditRadius
+                        }
+                    }
+
                     onWidthChanged: {
                         previewRenderer.height = previewRenderer.width * previewImageScalingFactor
                     }