libkwineffects: Make OffscreenQuickView opaque

OffscreenQuickView replaces the contents of the default scene. Nothing
will be repainted beneath it.

On the other hand, if the OffscreenQuickView contains translucent region,
there might be visual glitches. To prevent that, this change makes
scene effect views opaque.

src/libkwineffects/kwinquickeffect.cpp

@@ -84,7 +84,8 @@ bool QuickSceneEffectPrivate::isItemOnScreen(QQuickItem *item, Output *screen) c
 }
 
 QuickSceneView::QuickSceneView(QuickSceneEffect *effect, Output *screen)
-    : m_effect(effect)
+    : OffscreenQuickView(ExportMode::Texture, false)
+    , m_effect(effect)
     , m_screen(screen)
 {
     setGeometry(screen->geometry());