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:

* [x] minor simplification in `OrthogonalPath`
* [x] drop dependency on sklearn by including the dataset in vispy (#389).
* [x] ~~support for n-dimensional data points (instead of 4 dimensions max here), related to #390~~ (to be done later)

Close #389.

I would say it's important that we write vispy so that using float64 will
not lead to subtle bugs but either handle the cases correctly or throw
informative errors, since it's likely to be a common mistake. I think gloo
is programmed to give explicit errors about it. This being in an example
will give users the false impression that they need to cast color outputs...