Qt: Fix rare bug where covers get stuck in loading state

When quickly changing cover scale.

src/common/lru_cache.h

@@ -126,6 +126,13 @@ public:
       Evict(m_items.size() - m_max_capacity);
   }
 
+  template<typename F>
+  void Apply(const F& func)
+  {
+    for (auto& [key, value] : m_items)
+      func(const_cast<const K&>(key), value.value);
+  }
+
 private:
   void ShrinkForNewItem()
   {

src/duckstation-qt/gamelistwidget.cpp

@@ -278,6 +278,11 @@ void GameListModel::setCoverScale(float scale)
 
   m_cover_scale = scale;
 
+  // Invalidate all pending loads. This stops the case where the user changes the scale,
+  // then quickly changes back before the loads finish, resulting in the image never loading.
+  m_cover_pixmap_cache.Apply(
+    [](const std::string&, CoverPixmapCacheEntry& entry) { entry.scale = entry.is_loading ? 0.0f : entry.scale; });
+
   Host::SetBaseFloatSettingValue("UI", "GameListCoverArtScale", scale);
   Host::CommitBaseSettingChanges();
   updateCoverScale();