Disable Extensions: Volume should be independent

[echadwick-artist]

The disable control for Volume should be independent of Transmission and Diffuse Transmission, because it can interact with other extensions like Dispersion (and potentially other future extensions which interact with Volume).

In the Advanced Controls tab, if all extensions are disabled except KHR_materials_transmission, KHR_materials_volume, and KHR_materials_dispersion, and subsequently KHR_materials_transmission is disabled, then KHR_materials_volume is automatically disabled. This prevents Dispersion from using Volume. The same behavior occurs with KHR_materials_diffuse_transmission.

[emackey]

Volume & Dispersion can't have a visual effect without either transmission or diffuse transmission, which is why the logic was put there in the first place. Without one of those two, light cannot penetrate the surface, so the volumetric and dispersion properties don't make any difference. But, perhaps this bit of UI logic is overkill here.