force blend states to keepGoing

makes blend states keepGoing!

fixes a regression introduced when looking at keep going of states when deciding if to advance them. for blend states specifically mixing is done as well as animating animation instances, and if the mix value changes we need to apply this change.

@luigi-rosso struggled to build state machines into an automated test on the cpp side before. going to add one loading a `.riv` for this I think, at least to cover some basics here.

btw, I assume we also want this behaviour for direct blend states

https://2dimensions.slack.com/archives/CHMAP278R/p1677820699177779

Diffs=
2504d2ab2 force blend states to keepGoing (#4922)

.rive_head

@@ -1 +1 @@
-bc6c6f467b8f1e0f794be093a514768bd068088a
+2504d2ab24d258621fc89a0120c794f2b53a2ffc

lib/src/rive_core/animation/blend_state_instance.dart

@@ -27,18 +27,20 @@ abstract class BlendStateInstance<T extends BlendState<K>,
             .toList(growable: false),
         super(state);
 
-  bool _keepGoing = true;
   @override
-  bool get keepGoing => _keepGoing;
+  bool get keepGoing => true;
 
   @mustCallSuper
   @override
   void advance(double seconds, StateMachineController controller) {
-    _keepGoing = false;
     // Advance all the animations in the blend state
+    // NOTE: we are intentionally ignoring the animationInstances' keepGoing
+    // return value.
+    // Blend states need to keep blending forever, as even if the animation
+    // does not change the mix values may
     for (final animation in animationInstances) {
-      if (animation.animationInstance.advance(seconds) && !keepGoing) {
-        _keepGoing = true;
+      if (animation.animationInstance.keepGoing) {
+        animation.animationInstance.advance(seconds);
       }
     }
   }