Even if related, they don't have the same performance
impact.
To avoid any performance hit, we replace the Diffuse
by a Subsurface Closure for legacy EEVEE and
use the subsurface closure only where needed for
EEVEE-Next leveraging the random sampling.
This increases the compatibility with cycles that
doesn't modulate the radius of the subsurface anymore.
This change is only present in EEVEE-Next.
This commit changes the principled BSDF code so that
it is easier to follow the flow of data.
For legacy EEVEE, the SSS switch is moved to a
`radius == -1` check.
ea989ebf94