Allow showing anatomical slices in the background

[jhlegarreta]

Allow showing anatomical slices in the background while showing other data structures lying in the same volume in the foreground.

Is your feature request related to a problem? Please describe.
The feature request is not related to a problem.

Describe the solution you'd like
Given an arbitrary slice from a 3D volume (e.g. coming from MRI data), it might be useful to be able to show it as if it where in the background/last slice. This would be most useful when we have other data of interest (e.g. a collection of streamlines) that have parts before and past the slice of interest, and we would like to show the actor corresponding to such data of interest in the foreground, avoiding hiding/masking parts of it by the slice.

So imagine we have a 3D numpy array of shape (100, 100, 100) that we slice using the actor.slicer.display method at (None, None, 50), and we have a line, sphere or any other actor that needs to be plot together with the slice, but lies at slices that extend before and after the 50 index slice of the 3D volume (e.g. [40, 77]). We would like to avoid hiding/masking parts of the line, sphere or the relevant actor by the slice.

Describe alternatives you've considered
Although generating an additional fake volume repeating the slice of interest so that the last one can be picked could be proposed, it is a not very elegant/efficient to solve the issue. A plane/canvas at the last slice of the volume should rather be created and filled with the data of the desired slice.

Simply setting a value to the opacity of the slice, which is possible in the current FURY implementation, is not a solution to the feature request.

Additional context
A GitHub gist with a self-contained code and screenshots showing the issue are available in:
https://gist.github.com/jhlegarreta/8f901c4e6762c6402096bab009caf8ed

Thanks.