Additively blend terrain normals

Core set of changes was suggested by deepseek, I rewrote it. This fixes a lot of post-processing shaders.

The preexisting behavior just draws an arbitrary choice of texture's normals and is unusable. It's not even vaguely consistent with what terrain textures are where on the ground.

"But the normal won't be normalized" -- post-processing shaders for existing versions of openmw already have to renormalize normals in some cases (e.g. adding more specular reflections) because they're rounded to a u8 and have non-unit length so dot products and vector reflections get all messed up. Worst case scenario normals can be renormalized in a pre-postprocess cleanup pass, but that's future work, not current.

One question: should the writeNormals define be kept or removed?

Before and after: