New n-dimensional scatter plot example with GPU-based smooth projections

Not ready for merge, but you can already try it!

Things to be done:

* [ ] minor simplification in `OrthogonalPath`
* [ ] support for n-dimensional data points (instead of 4 dimensions max here), related to #390
* [ ] drop dependency on sklearn by including the dataset in vispy (#389)

I recently make `ColorArray` use `np.float32`, so you shouldn't need the `.astype(np.float32)`