Fix image masking and smoothing

Image masking with MultiEffect was not applying antialiasing at the edges
with the default settings.
The mipmap setting was ignored if the image source was changed (e.g. on
HeaderBar).  Added a hack to reset this AFTER the image has changed.

src/qml/FeedListDelegate.qml

@@ -166,7 +166,7 @@ Controls.ItemDelegate {
             imageSource: feed.cachedImage
             imageTitle: feed.name
             isLoading: feed.refreshing
-            absoluteRadius: Kirigami.Units.smallSpacing + borderWidth
+            absoluteRadius: Kirigami.Units.smallSpacing - borderWidth
         }
 
         Rectangle {

src/qml/ImageWithFallback.qml

@@ -26,6 +26,14 @@ Item {
     property bool imageResize: true
     property bool mipmap: false
 
+    // HACK: if mipmap has been set to true, then reset it AFTER image has
+    // changed, otherwise it will be ignored
+    onImageSourceChanged: {
+        if (root.mipmap) {
+            root.mipmap = true;
+        }
+    }
+
     Loader {
         id: imageLoader
         anchors.fill: parent
@@ -38,6 +46,8 @@ Item {
         source: imageLoader.item
         opacity: root.imageOpacity
         maskEnabled: true
+        maskThresholdMin: 0.5
+        maskSpreadAtMin: 0.5
         maskSource: ShaderEffectSource {
             width: imageLoader.width
             height: imageLoader.height