Preview-only Background Color

[mxsdev]

Description
Would be nice to have a separate preview/viewport background color setting which doesn't affect the render. This is particularly useful when you want to export with a transparent background (to add perhaps a more complex background in post), but still want to get an idea of how it will look roughly while editing.

Proposed solution
Add an extra Video Setting under preview for "background". When empty, this will default to the General background, and when non-empty, this will become the background color for the viewport. This will not affect the render result at all - that will still use the background under "General".

Considered alternatives
Alternative is to change the background every time you need to render and then change it back for preview purposes. For scenarios where there are many scenes to render, this becomes tiresome quickly.

Additional context
Happy to make a PR for this, it seems fairly straight-forward :)

[aarthificial]

I think it's a great idea. But we should stay consistent with other settings and make the background setting separate for the preview and for the rendering without any inheritance/default behavior (for now).

Having a way to override the settings when previewing is an interesting idea but it should be implemented in a separate PR and affect other similar settings like FPS or Scale.