Add early return when setting transparent_bg

[Sauermann]

Fix memory leak on Mac.

circumvents #90030

Since I can't reproduce the problem of the linked issue, It would be great, if someone else could test, if the memory leak on MacOs is resolved.

[BastiaanOlij]

Probably something where we want to dig a little deeper to see why it's leaking. It's still possible we're leaking when changing the background mode and this is just a stopgap.

That said, recreating our buffers just because the background mode is touched without it actually changing seems overkill so this change could be justified for that reason.

[Sauermann]

Sounds good to me. Lets keep this PR open until after the actual reason for the memory leak is identified. It is very likely, that I will not dig deeper into that topic in the near future.

Edit: There is a MRP available to reproduce the memory leak with this PR merged.

[Calinou]

This makes sense to do for efficiency reasons.

[clayjohn]

I have a feeling that the memory leak comes from MoltenVK, since it is not leaking on non-Apple devices.

I would go ahead and merge this since it is useful anyway. And then modify the MRP to instead flip the transparent_bg mode each frame

[akien-mga]

Thanks!