The diffuseTransmissionColor and baseColor need to be applied twice for volumes

The baseColor defines which color passes through each surface in KHR_materials_transmission.
The diffuseTransmissionColor does the same for KHR_materials_diffuse_transmission.
Since these colors can be textured, one can have a different color on the surface of the incoming light and the surface of the outgoing light.

We need to create a new render pass which renders the backside of the meshes.

Here is a test asset for transmission using a black-white base color texture:

[Volume_Texture_Test.zip](https://github.com/user-attachments/files/23604821/Volume_Texture_Test.zip)

Start by loading the linked Volume_Texture_Test.zip asset in the sample viewer and inspect the existing transmission and diffuse-transmission rendering paths. Implement the requested backside render pass so baseColor and diffuseTransmissionColor can differ between incoming and outgoing surfaces, then verify the black-white texture asset renders with the expected volume transmission.

Written by the indexing model from the issue text.