Fixes to animation graph evaluation (#15689)

# Objective

Fix a couple of substantial errors found during the development of
#15665:
- `AnimationCurveEvaluator::add` was secretly unreachable. In other
words, additive blending never actually occurred.
- Weights from the animation graph nodes were ignored, and only
`ActiveAnimation`'s weights were used.

## Solution

Made additive blending reachable and included the graph node weight in
the weight of the stack elements appended in the curve application loop
of `animate_targets`.

## Testing

Tested on existing examples and on the new example added in #15665.

crates/bevy_animation/src/lib.rs

@@ -1102,7 +1102,7 @@ pub fn animate_targets(
  };
 
  match animation_graph_node.node_type {
- AnimationNodeType::Blend | AnimationNodeType::Add => {
+ AnimationNodeType::Blend => {
  // This is a blend node.
  for edge_index in threaded_animation_graph.sorted_edge_ranges
  [animation_graph_node_index.index()]
@@ -1123,6 +1123,27 @@ pub fn animate_targets(
  }
  }
 
+ AnimationNodeType::Add => {
+ // This is an additive blend node.
+ for edge_index in threaded_animation_graph.sorted_edge_ranges
+ [animation_graph_node_index.index()]
+ .clone()
+ {
+ if let Err(err) = evaluation_state
+ .add_all(threaded_animation_graph.sorted_edges[edge_index as usize])
+ {
+ warn!("Failed to blend animation: {:?}", err);
+ }
+ }
+
+ if let Err(err) = evaluation_state.push_blend_register_all(
+ animation_graph_node.weight,
+ animation_graph_node_index,
+ ) {
+ warn!("Animation blending failed: {:?}", err);
+ }
+ }
+
  AnimationNodeType::Clip(ref animation_clip_handle) => {
  // This is a clip node.
  let Some(active_animation) = animation_player
@@ -1175,7 +1196,7 @@ pub fn animate_targets(
  continue;
  };
 
- let weight = active_animation.weight;
+ let weight = active_animation.weight * animation_graph_node.weight;
  let seek_time = active_animation.seek_time;
 
  for curve in curves {
@@ -1323,6 +1344,20 @@ impl AnimationEvaluationState {
  Ok(())
  }
 
+ /// Calls [`AnimationCurveEvaluator::add`] on all curve evaluator types
+ /// that we've been building up for a single target.
+ ///
+ /// The given `node_index` is the node that we're evaluating.
+ fn add_all(&mut self, node_index: AnimationNodeIndex) -> Result<(), AnimationEvaluationError> {
+ for curve_evaluator_type in self.current_curve_evaluator_types.keys() {
+ self.curve_evaluators
+ .get_mut(curve_evaluator_type)
+ .unwrap()
+ .add(node_index)?;
+ }
+ Ok(())
+ }
+
  /// Calls [`AnimationCurveEvaluator::push_blend_register`] on all curve
  /// evaluator types that we've been building up for a single target.
  ///