Don't render disabled nodes

[ArcturusEmrys]

When a node is disabled, it does not render (but it can still be used as a mask in disabled state).

Cases I haven't investigated yet:

• Children of disabled nodes - if I disable them in Inochi Session, they disappear. I haven't made this explicitly cascade the disabled value to children, and the renderer gets a flattened list, so...
• Children of a disabled mask - they should still contribute to the mask, but I haven't checked.

[ArcturusEmrys]

Some updates on this after testing:

• The only valid masks are Parts. Inochi Creator will not allow you to select any other object as a mask source... except MeshGroups, which fail to render and render the target invisible.
• Disabled state doesn't matter for masking, even transitively.
• Disabled state DOES cascade to children. This is actually the only part of rendering that cares about the node hierarchy, so I had to do some really awful tree traversal for every render step.

As part of making a testcase for this, I also found a number of bugs... in Inochi Creator/Session, completely unrelated to Inox:

• MeshGroups can be selected as a mask source. As mentioned above, this is not a valid mask source, so doing so hides the masked object completely.
• "Prune unused nodes" export will happily write out a file with masks referencing nodes that don't exist.
  • When Inox reads such a file, it panics.