vol*: interactive 3D label painting

[alisterburt]

This is a high value feature for volume rendering identified in #1086

3D vis is cool, 3D annotation is harder

For ML applications generating 3D segmentations is often a bottleneck and the tools that can do this well are mostly closed.

We should aim to enable the painting of labels into 3D volumes in mol*. This can be enabled by painting onto 2D images, either linked orthoviews (#1094) or oblique planes (#1092).

Where this is done well:

• in napari you can paint in 3D on 2D slices
• in amira/avizo/imaris you have a similar workflow on orthoviews

Is there any experience in adding painting tools over images in mol*?