Fix: Decal 'Normal Fade' feature appears incorrect on scaled meshes

[techiepriyansh]

Fixes #56590.

Before:

After:

[akien-mga]

This would need a rebase as the files moved. The code still seems to be the same as before this PR so the fix might still be relevant, though I'm not familiar enough with decals to assess whether #56590 is still reproducible (CC @Calinou).

[Calinou]

The code still seems to be the same as before this PR so the fix might still be relevant, though I'm not familiar enough with decals to assess whether #56590 is still reproducible (CC @Calinou).

See #56590 (comment).

[clayjohn]

While this fixes the bug, it also introduces a change in behaviour. normal_interp contains the geometric normal from the vertex function while normal contains the transformed normal from the user including the normal_map.

If we want to keep the current behaviour the same, we should normalize normal_interp. Everywhere else in the shader that normal_interp is used it is normalized first.

At any rate, we can merge this after 4.0. I would like to discuss whether it would be preferable to include normal maps in the normal fading behaviour.

[Calinou]

I would like to discuss whether it would be preferable to include normal maps in the normal fading behaviour.

This is probably desired for decals made of "particles", such as patches of sand or snow that are sprinkled on top of another surface.