Clamp normal when calculating 2D lighting to avoid artifacts

[clayjohn]

Fixes: #76233

This is the same fix that was applied to 3.3 in #44293. The Godot 4 2D rendering must have diverged before #44293 was made.

Before:

After:

[clayjohn]

I'm still looking into this by the way. The discussion in #44285 never fully settled. It seems like this change may only be a partial fix for the worst of it

Nevermind, this should be good to go

[akien-mga]

Thanks!