[Feature] support visualising meshes in just one hemisphere.

**Is your feature request related to a problem? Please describe.**
Showing the mesh in just one hemisphere is a common use case.

**Describe the solution you'd like**
Being able to do the above through the GUI

**Describe alternatives you've considered**
Pre-compute hemi-meshes and provide them through the atlas API. Would make download slower though.

**Additional context**
[Discussed on a forum post](https://forum.image.sc/t/brainrender-napari-change-mesh-color-and-display/96798/)

Start by tracing the GUI path that displays atlas meshes and how the atlas API supplies them. Determine how hemisphere selection should be exposed and rendered, considering the proposed precomputed hemi-mesh alternative. Done means the GUI supports displaying a single hemisphere while retaining the existing mesh-display use case.

Written by the indexing model from the issue text.