Mixing multiple volume phase closures can make world background
lighting slightly too bright. The phase-sampled path stores the PDF of
only the selected closure, while the background-sampled path evaluates
the marginal mixture PDF. The resulting MIS weights are inconsistent.
Store the marginal PDF over all phase closures for forward MIS. When
volume path guiding is active, combine it with the guiding PDF so the
stored PDF remains equal to the actual sampling distribution.
Pull Request: https://projects.blender.org/blender/blender/pulls/161690
