wall and floor planes

## Use Case

In order to ground the scene and give dimensional reference, it can be useful to have walls or floor planes that receive shadows at the X/Y/Z extents of the volume

## Solution

Ideally:
User can select which plane to put a wall. User can select color of wall. Walls will only be inward-facing so they never block camera view. The walls will not be part of the volume data but will be pathtraced.

## Alternatives

A previous hackish alternative was to put a single plane of high intensity values into the volume data itself at the dimensional extent (floor plane). However, this overwrites data in a particular channel and can't be colored independently. The renderer only handles 4 channels in volume data so it would be expensive to make it be a separate channel -- you'd only have 3 proper data channels left to display -- but possible.