Add a binding to control opacity

[githubkusi]

Context
F3D support --opacity to control the mesh opacity. While this is a great feature, it is not possible to modify the opacity interactively. Add a bind to do so would make F3D more reactive.

Improve suggestion

1. Add a dedicated command to increase/decrease opacity (see light intensity command for inspiration)
2. Add a dedicated bind to call that new command. Ctrl + T to decrase and Shift + T to increase comes to mind
3. Add dedicated tests

[Meakk]

Sounds like a good idea to me.
I think it will probably be addressed in F3D 3.0 with the interactor rework or at least the interactive console. Right @mwestphal ?

[mwestphal]

definitely!

[mwestphal]

need: #443

[mwestphal]

This can now be adressed!

[mwestphal]

I've improved the issue so it can be contributed more easily.

[19jrushlow]

First-time contributor here, I would like to work on this if possible.

[mwestphal]

Great, I've just assigned it to you.

Please join our discord if you can, more easy to discuss and help you when needed :)
https://discord.f3d.app

[19jrushlow]

@mwestphal should this issue be marked as resolved now?

[mwestphal]

Yes!