Add a helper to visualize bones in the scene

This is the helper that was used to solve the issue #4515. Screenshots were attached near the end of that PR. `scaleRatio` is meant to be used if the `SkinnedMesh` has a scale set. For example, the model used in that example was rendered with `@model.scale.x = @model.scale.y = @model.scale.z = 0.005`.