Fix Specular Blinn function

[clayjohn]

I noticed an issue with the Specular Blinn code path while working on a fix to the vertex-lit code path. Previously our normalized blinn function was divided by 4.0 * cNdotV * cNdotL. I am not sure why that term was included. It causes a weird circular artifact to appear, and it causes the specular lobe to slide around the entire mesh. None of the normalized-blinn NDFs and RDFs I could find used that term (not even Ogre3D's). However, I found that one of the current popular RDFs the modified-Blinn uses pow(cNdotH, shininess) * cNdotL;.

Using this change and removing the above scaling factor results in a much nicer Blinn Specular. The specular lobe looks a lot like the SCHLICK_GGX, only it is a little smaller on smooth materials and a little brighter on rough materials.

Because vertex-lit materials are forced to use Blinn Specular, it is especially important that it look nice.

This also resolves another bug I found where IBL was being scaled too dark on vertex-lit materials.

MASTER
Blinn on left, GGX on right

This PR
Blinn on left, GGX on right

Fixes: #33543

[akien-mga]

Didn't you add this extra scope a few commits ago in another PR? :)

[clayjohn]

Yes, but it made the code messier, and it is unnecessary now that I removed the environment scaling from the vertex_lighting code path

[akien-mga]

Previously our normalized blinn function was divided by 4.0 * cNdotV * cNdotL. I am not sure why that term was included. It causes a weird circular artifact to appear, and it causes the specular lobe to slide around the entire mesh. None of the normalized-blinn NDFs and RDFs I could find used that term (not even Ogre3D's).

Seems like it was added by @reduz in 65fd37c. From a quick look at this commit it looks like he also added it for Phong, might be worth reviewing that too.

BTW, I did a very quick test scenes, probably not with the best environment to see specular effects, but it seems that Specular Mode SchlickGXX and Disable give the exact same output.

[clayjohn]

Yea, the Phong lighting definitely appears strange, but it is not as obviously incorrect as the blinn is.

As for the specular modes, I fixed that in #33694, it just needs to be merged. :) The important part for that bug is the change to material.h. I could remove that part from the PR and make another one.

[akien-mga]

I could remove that part from the PR and make another one.

Might be worth doing since there was no clear OK from @reduz on the .glsl changes in #33694.

[reduz]

Seems good, please do a similar PR for the Vulkan brach

[akien-mga]

Thanks!