Fix InlineDictionary Behavior for Single Item Reassignment

Previous Behavior:

- When the `Set` method was called, it ignored the `overwrite` parameter if the dictionary contained only one item.
- This caused the `Set` method to behave like the `Add` method, leading to unexpected behavior in composition animations.
- Specifically, the second composition animation would not play because it could not replace the first animation. Instead, it was added after the first animation, preventing it from being executed.

Updated Behavior:

- The `InlineDictionary` now respects the `overwrite` parameter even when it contains only one item.
- This ensures that the second composition animation can overwrite the first one, allowing it to play correctly.

src/Avalonia.Base/Utilities/SmallDictionary.cs

@@ -77,7 +77,15 @@ void SetCore(TKey key, TValue value, bool overwrite)
         }
         else
         {
-            // We have a single element, upgrade to array
+            // We have a single element, check if we should update the value.
+            if((TKey)_data == key && overwrite)
+            {
+                _data = key;
+                _value = value;
+
+                return;
+            }
+            // If we do not replace it, upgrade to array.
             arr = new KeyValuePair[6];
             arr[0] = new KeyValuePair((TKey)_data, _value);
             arr[1] = new KeyValuePair(key, value);