fix screenshot capture: hide from wallpaper, refresh on each lock

shell/modules/ScreenCapture.qml

@@ -23,20 +23,25 @@ PanelWindow {
         id: _capture
         anchors.fill: parent
         captureSource: root.screen
+        visible: false
     }
 
     Connections {
         target: S.ScreenshotService
         function onCaptureRequested() {
             _capture.captureFrame();
+            _grabTimer.start();
         }
     }
 
-    Connections {
-        target: _capture
-        function onHasContentChanged() {
-            if (_capture.hasContent)
+    // Brief delay for the frame to arrive, then grab
+    Timer {
+        id: _grabTimer
+        interval: 50
+        onTriggered: {
+            _capture.visible = true;
             _capture.grabToImage(result => {
+                _capture.visible = false;
                 S.ScreenshotService.store(root.screen.name, result);
             });
         }