Fix SDFGI being used in unshaded debug draw

[tactical-fluke]

fixes #62758

Simply disables the using of SDFGI if the viewport draw mode is unshaded

Before:

After:

[tactical-fluke]

this feels like a bit of a hack, and like it should belong elsewhere. It seems to work, but I'm not entirely sure on what cases I should test.

Moreover, I'd appreciate if someone far more familiar with the code base could point me potentially towards a better place to fix this, if there is one :)

[Calinou]

this feels like a bit of a hack, and like it should belong elsewhere. It seems to work, but I'm not entirely sure on what cases I should test.

Technically, this approach is more correct as it'll also improve performance (as opposed to skipping SDFGI sampling in the shader, which will not skip SDFGI generation).

See #89253 which took a similar approach (with the original approach being split to #89235).

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉