Fix IBL specular persisting when specular disabled

[clayjohn]

Fixes: #33616

SPECULAR_DISABLED wasn't affecting IBL specular. So in PBR scenes, there were still specular highlights even when specular was disabled.

[reduz]

Not sure about this, it seems to me that the problem in the original issue is something else and it should be properly researched.

[clayjohn]

From what I understand, the problem the user is having is that they are seeing specular highlights even when specular is disabled.

They also think that setting "specular" to 0 should reduce the effect of specular highlights. But they are misunderstanding what the specular parameter does. fracteed does a nice job of explaining what the slider is for.

From a design perspective. We are giving the users the ability to disable specular, but it only disables specular from lights, not from IBL. The user is trying to disable all specular but is seeing no change when in an IBL-only scene. They also mistake fully-rough specular for specular being turned off.

[clayjohn]

Reduz and I discussed this over IRC. We concluded the best option would be to clarify that the current Specular Mode settings is actually a "Specular Lobe Mode" and to add a further flag that disables specular reflections.

It makes sense to treat the specular lobe differently from specular reflections. Once the Vulkan branch has been merged into master I will update this PR to reflect the above decision.

[aaronfranke]

@clayjohn Vulkan has been merged into master, so you can update this PR now.

[akien-mga]

Superseded by #41415?

[clayjohn]

@akien-mga not quite. I will edit this to expose a disable-specular property to StandardMaterial3D

[clayjohn]

Superseded by #63587