[feature] Add clipping plane support in Volume Rendering

Hi, @NicolasRannou . I want to clip the model by a plane. I found that the material of volume is 'MeshStandardMaterial' in your demo #2 which clip brain. Using 'MeshStandardMaterial' on volume can be success to clip the model in my demo.
However, the material of volume in demo #6 is 'ShaderMaterial'. I add the attribute of clippingPlanes on the material is invaild. I guess some errors occur on the 'ShaderMaterial'.
I want to know how to clip the volume in demo #6. Looking forward to your reply.

Start with demo #6 and compare its ShaderMaterial setup with the MeshStandardMaterial used in demo #2. Investigate how clippingPlanes are handled for the shader-based volume material and identify the required shader or material changes. Done means a clipping plane reliably clips the volume in demo #6.

Written by the indexing model from the issue text.