CmdPal: Improvements and fixes for icon loading (#45460)

## Summary of the Pull Request

This PR is a follow-up for Icon cache:

- Adds decoding and rasterization limit (by width) to reduce memory
usage and improves throughput (noticeably)
- Fixes timing issue when setting padding for font icons
- Resolves race condition in IconBox caused by incorrect guard

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## PR Checklist

- [x] Closes: #45460
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/modules/cmdpal/Microsoft.CmdPal.UI/Controls/IconBox.cs

@@ -16,12 +16,12 @@ namespace Microsoft.CmdPal.UI.Controls;
 /// </summary>
 public partial class IconBox : ContentControl
 {
+    private const double DefaultIconFontSize = 16.0;
+
     private double _lastScale;
     private ElementTheme _lastTheme;
     private double _lastFontSize;
 
-    private const double DefaultIconFontSize = 16.0;
-
     /// <summary>
     /// Gets or sets the <see cref="IconSource"/> to display within the <see cref="IconBox"/>. Overwritten, if <see cref="SourceKey"/> is used instead.
     /// </summary>
@@ -62,6 +62,12 @@ public partial class IconBox : ContentControl
             {
                 Refresh();
             }
+#if DEBUG
+            if (_sourceRequested?.GetInvocationList().Length > 1)
+            {
+                Logger.LogWarning("There shouldn't be more than one handler for IconBox.SourceRequested");
+            }
+#endif
         }
         remove => _sourceRequested -= value;
     }
@@ -102,9 +108,12 @@ public partial class IconBox : ContentControl
         if (Source is FontIconSource fontIcon)
         {
             fontIcon.FontSize = _lastFontSize;
+            UpdatePaddingForFontIcon();
         }
     }
 
+    private void UpdatePaddingForFontIcon() => Padding = new Thickness(Math.Round(_lastFontSize * -0.2));
+
     private void OnActualThemeChanged(FrameworkElement sender, object args)
     {
         if (_lastTheme == ActualTheme)
@@ -150,10 +159,7 @@ public partial class IconBox : ContentControl
 
     private void Refresh()
     {
-        if (SourceKey is not null)
-        {
-            UpdateSourceKey(this, SourceKey);
-        }
+        UpdateSourceKey(this, SourceKey);
     }
 
     private static void OnSourcePropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
@@ -170,8 +176,10 @@ public partial class IconBox : ContentControl
                 self.Padding = default;
                 break;
             case FontIconSource fontIcon:
+                self.UpdateLastFontSize();
                 if (self.Content is IconSourceElement iconSourceElement)
                 {
+                    fontIcon.FontSize = self._lastFontSize;
                     iconSourceElement.IconSource = fontIcon;
                 }
                 else
@@ -190,7 +198,7 @@ public partial class IconBox : ContentControl
                     self.Content = elem;
                 }
 
-                self.Padding = new Thickness(Math.Round(self._lastFontSize * -0.2));
+                self.UpdatePaddingForFontIcon();
 
                 break;
             case BitmapIconSource bitmapIcon:
@@ -206,10 +214,12 @@ public partial class IconBox : ContentControl
                 self.Padding = default;
 
                 break;
+
             case IconSource source:
                 self.Content = source.CreateIconElement();
                 self.Padding = default;
                 break;
+
             default:
                 throw new InvalidOperationException($"New value of {e.NewValue} is not of type IconSource.");
         }
@@ -233,10 +243,10 @@ public partial class IconBox : ContentControl
             return;
         }
 
-        Callback(iconBox, sourceKey);
+        RequestIconFromSource(iconBox, sourceKey);
     }
 
-    private static async void Callback(IconBox iconBox, object? sourceKey)
+    private static async void RequestIconFromSource(IconBox iconBox, object? sourceKey)
     {
         try
         {
@@ -256,17 +266,12 @@ public partial class IconBox : ContentControl
             // list virtualization situation, it's very possible we
             // may have already been set to a new icon before we
             // even got back from the await.
-            if (eventArgs.Key != sourceKey)
+            if (!ReferenceEquals(sourceKey, iconBox.SourceKey))
             {
                 // If the requested icon has changed, then just bail
                 return;
             }
 
-            if (eventArgs.Value == iconBox.Source)
-            {
-                return;
-            }
-
             iconBox.Source = eventArgs.Value;
         }
         catch (Exception ex)