ROI clipping doesn't clip everything

## Description

When a ROI is slicing through the volume, normal vectors are still being computed as if the volume was whole. As a result, interiors of solid structures seem hollow when viewed as cross section. Also, scattered rays keep sampling the volume data that was clipped out.

## Expected Behavior

clipping the volume should truly remove that data from all sampling.

## Notes

per-channel clipping and truly zeroing out the gpu volume data would do the trick, instead of using the AABB only for the first ray-box hit. Also, testing the AABB on every sample would work as well (but slower?)