migrate some animation helper classes

lib/animations/morph_animation_data.dart

@@ -0,0 +1,31 @@
+//
+// Frame weights for the morph targets specified in [morphNames] attached to mesh [meshName].
+// morphData is laid out as numFrames x numMorphTargets
+// where the weights are in the same order as [morphNames].
+// [morphNames] must be provided but is not used directly; this is only used to check that the eventual asset being animated contains the same morph targets in the same order.
+//
+import 'dart:typed_data';
+
+class MorphAnimationData {
+  final String meshName;
+  final List<String> morphNames;
+
+  final Float32List data;
+
+  MorphAnimationData(
+      this.meshName, this.data, this.morphNames, this.frameLengthInMs) {
+    assert(data.length == morphNames.length * numFrames);
+  }
+
+  int get numMorphWeights => morphNames.length;
+
+  int get numFrames => data.length ~/ numMorphWeights;
+
+  final double frameLengthInMs;
+
+  Iterable<double> getData(String morphName) sync* {
+    for (int i = 0; i < numFrames; i++) {
+      yield data[i * numMorphWeights];
+    }
+  }
+}