qml: do not use radius if it is smaller than 4dp

- Too small radius is hard to notice, so it is not
  worth to have it over regular image.
- When there is no radius, anti-aliasing is not
  necessary, so if the source image is opaque,
  blending would not be necessary which makes
  the batch renderer do better batching.

modules/gui/qt/style/VLCStyle.qml

@@ -89,10 +89,10 @@ QtObject {
 
     readonly property int listAlbumCover_height: MainCtx.dp(32, scale)
     readonly property int listAlbumCover_width: listAlbumCover_height * 16.0/9
-    readonly property int listAlbumCover_radius: MainCtx.dp(3, scale)
+    readonly property int listAlbumCover_radius: 0
     readonly property int trackListAlbumCover_width: MainCtx.dp(32, scale)
     readonly property int trackListAlbumCover_heigth: MainCtx.dp(32, scale)
-    readonly property int trackListAlbumCover_radius: MainCtx.dp(2, scale)
+    readonly property int trackListAlbumCover_radius: 0
 
     readonly property int tableCoverRow_height: Math.max(listAlbumCover_height, fontHeight_normal) + margin_xsmall * 2
     readonly property int tableRow_height: fontHeight_normal + margin_small * 2

modules/gui/qt/widgets/qml/DragItem.qml

@@ -452,7 +452,7 @@ Item {
             Rectangle {
                 id: bg
 
-                radius: coverRepeater.count > 1 ? dragItem.coverSize : VLCStyle.dp(2, VLCStyle.scale)
+                radius: coverRepeater.count > 1 ? dragItem.coverSize : 0.0
                 anchors.fill: parent
                 color: theme.bg.primary