feat: Add clearSnapshot function (#2897)

The Snapshot mixin was missing a function to clear the snapshot. This is
useful for when you want to invalidate the picture, but don't want to
immediately pre-render the image yet.

packages/flame/lib/src/components/mixins/snapshot.dart

@@ -69,6 +69,12 @@ mixin Snapshot on PositionComponent {
     return _picture!;
   }
 
+  /// clear the current snapshot. will trigger the creation of a new snapshot
+  /// next time renderTree is called.
+  void clearSnapshot() {
+    _picture = null;
+  }
+
   @override
   void renderTree(Canvas canvas) {
     if (renderSnapshot) {