Add the possibility to process the model's geometry

[gabotechs]

As an alternative to #37, let the users perform an arbitrary post-processing to the loaded geometry.

The post-processing callback is just one more parameter in the ModelProps, and it can be used like this:

<StlViewer
  modelProps={{
     geometryProcessor: (geometry) => LoopSubdivision.modify(geometry, 3, { })
  }}
/>

[u-u-z]

greate things ~