implement bounding box in pathtrace mode

## Use Case

pathtrace mode never had bounding box lines implemented

## Solution

strategy: draw backfacing box lines, then draw pathtrace volume blended on top, then draw frontfacing box lines? might require fake depth-buffer-filling draw pass as well.

## Alternatives

The pathtrace render pass is currently fullscreen ray tracing. It would be interesting to modify the pathtrace volume draw calls to draw a 3d box and only pathtrace the pixels of the box covering the volume data. This has implications about how we render objects (typically a ray tracer traces rays into a whole scene full of stuff - it doesn't draw objects one by one).