bind method
skeleton - Skeleton created from a Bones tree.
bindMatrix - Matrix4 that represents the base
transform of the skeleton.
Bind a skeleton to the skinned mesh. The bindMatrix gets saved to .bindMatrix property and the .bindMatrixInverse gets calculated.
Implementation
void bind(Skeleton skeleton, [Matrix4? bindMatrix]) {
  this.skeleton = skeleton;
  if (bindMatrix == null) {
    updateMatrixWorld(true);
    this.skeleton!.calculateInverses();
    bindMatrix = matrixWorld;
  }
  this.bindMatrix!.setFrom(bindMatrix);
  bindMatrixInverse..setFrom(bindMatrix)..invert();
}