Add Keep Alive property to background filter

[men232]

This property ensures that the background filter remains active even when switching to another scene in OBS.

It is especially useful when you have a separate webcam scene with a background removal filter and use a source clone in another scene.

[royshil]

@men232 can yo plz run clang-format on src/background-filter.cpp?

[umireon]

Stalled?

[men232]

@royshil done, please review

[umireon]

I suggest that this option should be inverted as "Stop filter when source is inactive" and having checked by default.
When this checkbox is off, this filter will continue processing even if the direct parent is not shown.
@royshil How do you think?

[royshil]

Yes the inverse makes sense @umireon

[umireon]

Invert the logic of keepalive property and name it in an easy-to-understand manner, please.

[umireon]

Stalled?