Improve Ui performance when loading models with many morph targets

[slimbuck]

The UI is very sluggish when loading models with hundreds of morph targets.

To fix this we stopped updating morph sliders in the UI when a model is animating. However users have reported that the sliders are actually useful when debugging etc.

We need to understand why updating morphs are so slow and fix it.

[willeastcott]

@slimbuck Do you have a glTF file with which this can be tested/profiled?