Ensure optional CopyEffects variants are loaded last.

[BastiaanOlij]

This is a quick fix for #84841

There is an issue with our shader cache when there are disabled variants, it does not seem to deal with these properly (though this seems to be a recently introduced issue, possibly due to shader groups being added in 4.2).
For the most part this isn't a problem as the missing variants end up being recompiled and the cache fixes itself. The exception are our copy effects where a new copy type was added after the optional variants.
There is a more serious bug in the shader cache that causes all variants to be deleted (even the ones loaded properly) but in such a way that the Godot can no longer compile new versions.

This PR works around the problem by moving this new variant before the optional variants. This fixes the immediate problem.

We will need to fix the cache logic at some point but that will be a 4.3 fix.

Bugsquad edit:

• Fixes Toggling XR/Shaders/Enabled caused errors spam when using WorldEnvironment #84841

[BastiaanOlij]

Just for future reference, we'll need to investigate this further. But I think the problem is that disabled variants are simply not added to the cache at all. So now it's trying to load the COPY_TO_FB_SET_COLOR variant as if it's the COPY_TO_FB_MULTIVIEW.
We'll probably need to save something into the cache to indicate that slow contains a disabled variant, so that COPY_TO_FB_SET_COLOR is saved in the correct slot.

[akien-mga]

Looks good to me.

[clayjohn]

Seems fine to me. As-is, I think this is a very low risk change.

That said, I think it is a band-aid to an underlying problem with how we deal with disabling/enabling XR variants. So we should probably revisit this after 4.2 releases

[akien-mga]

Thanks!