Voxel artifacts when camera direction is parallel to clipping angle

[lilleyse]

When the camera direction is parallel to any of the clipping angles the shape intersection code in the shader breaks down. This was fixed for cylinder wedges less than 180 degrees in 48d4790 and would require a similar but slightly more complicated fix in intersectRegularWedge. I only tested this for cylinders, but it's possible that other shapes that do half-space or plane intersection will have similar issues. In general the intersection code does not robustly handle cases where the ray origin is on the plane, parallel to the plane, or both.

In the gif below, notice how moving the camera fixes the problem. That is because the camera is no longer on the plane.

Sandcastle